The rules are behind walls of text and part of a rules salad, but

A Creatures Doesn't Get More Than Four Attacks from Its Base Attack Bonus

In the sidebar Behind the Curtain: A Limit to Attacks and Saves, the Dungeon Master's Guide says, in part,

After 20th level, your base attack bonus never improves. You gain epic bonuses and other bonus on your attack roll, but these don't ever increase your base attack bonus and thus never grant you additional attacks.

This limit [to a creature's number of attacks] doesn't apply to the base attack bonus derived strictly from a monster’s Hit Dice. For instance, a titan with 21 HD using the advancement rules in the Monster Manual has a base attack bonus of +21. Thus, another cap exists: A high base attack bonus never grants a creature more than four attacks with any given weapon using the full attack option.

A naztharune rakshasa (MM3 196-7) Rogue 8/Scout 6 has a Base Attack Bonus of +21, making the creature's attack routine solely because of its Base Attack Bonus +21/+16/+11/+6.

From its 11 Hit Dice from outsider it gets +11 Base Attack Bonus, from its 8 levels of rogue it gets +6 Base Attack Bonus, and from its 6 levels of scout it gets +4 Base Attack Bonus. I've added all of these together to get the creature's Base Attack Bonus; this process may differ among DMs, though, because the rules are confusing and contradictory.

"How'd You Figure That?"

A naztharune rakshasa begins play with 11 Hit Dice and Level Adjustment +5. Thus a naztharune rakshasa Rogue 8/Scout 6 has an Effective Character Level of 30, 25 Hit Dice, and 14 character levels.

"What's That Mean?"

There's this:

To determine the effective character level (ECL) of a monster character, add its level adjustment to its racial Hit Dice and character class levels. Use ECL instead of character level to determine how many experience points a monster character needs to reach its next level. Also use ECL to determine starting wealth for a monster character.

When determining what sort of challenges the creature should face, how much wealth the creature should have, and how much XP it takes the creature to advance to the next level, the creature uses his ECL of 30.

Then there's this:

The creature’s Hit Dice equal the number of class levels it has plus its racial Hit Dice. [...] A monster’s total Hit Dice, not its ECL, govern its acquisition of feats and ability score increases.

When determining anything based on the creature's Hit Dice--including feats (the creature can pick epic feats, by the way) and ability score increases as well as some special abilities (e.g. the naztharune rakshasa Rogue 8/Scout 6 has +13d6 sneak attack from its extraordinary ability sneak attack without adding in additional sneak attack dice from the levels of rogue)--the creature has 25 Hit Dice.

Finally, there's this:

Epic characters—those whose character level is 21st or higher—are handled slightly differently from nonepic characters. While epic characters continue to receive most of the benefits of gaining levels, some benefits are replaced by alternative gains.

When determining anything else based on character level, a naztharune rakshasa Rogue 8/Scout 6 is a level 14 character. In theory, the creature can gain 6 more character levels before he has to play by the rules presented in the Dungeon Master's Guide on Table 6-18: Epic Save and Epic Attack Bonuses (206).

"Why Only 'In Theory'?"

The Version 3.5 Main D&D FAQ confuses things mightily. First it says

Use the character’s ECL to determine starting equipment and how the character earns and benefits from experience (including when he gains an epic attack and save bonus; DMG 209), as noted on page 5 of SS. Use the actual character level for everything else. (2)

Then, later, it says

Question: When is a monster character considered epic level? Do you “go epic” when your total class levels equal 20 or when your total Hit Dice equal 20? Is a monster character eligible for epic-level feats (such as Epic Toughness) when its character level is 21+ or when its ECL is 21+?

Answer: A monster becomes an epic-level character when its character level hits 21, just like any other character. A monster’s character level equals its racial Hit Dice + class levels. (See the second sidebar on page 25 of the [Epic Level Handbook].) A creature’s ECL has no effect on when it becomes an epic character, although once it becomes an epic character, its ECL continues to affect how much experience it earns and when it can add a new level. (3)

You'll note that those are contradictory and but one page apart. And, oddly, they're both apparently inaccurate. The the August 2012 printing of the Monster Manual (identical to the 2003 printing in this way) has the frost giant jarl (122-3), a creature with an ECL 26, 22 Hit Dice, and 8 class levels of a class that grants +1 Base Attack Bonus per level. Its printed Base Attack Bonus is +18.

• A frost giant jarl who obeys the first rules described in the FAQ has Base Attack Bonus of +15.
• A frost giant jarl who obeys the second rules described in the FAQ has a Base Attack Bonus of +17.
• A frost giant jarl who adds his racial Base Attack Bonus to his class levels' Base Attack Bonuses has a Base Attack Bonus of +18.

It appears, then, that despite the FAQ's contradictions, when it comes down to it, and actual creatures are printed, that Base Attacks Bonus (in addition to, one assumes, saving throw bonuses and other class features) continues to accrue, ignoring both ECL and racial Hit Dice with regard to Table 6-18: Epic Save and Epic Attack Bonuses (DMG 206), until post-character level 20.

I mean, I guess so, anyway. There's probably no way to ever really know for sure. But that way's fine. It's playable, makes things easy to determine, and at least agrees with one primary source, the Monster Manual. Trying to balance epic play is absurd anyway--as long as all the PCs are the same ECL, I doubt there'll be complaining (at least until someone starts casting epic spells).

So you know, a naztharune rakshasa Rogue 8/Scout 6 could have any of the following Base Attack Bonuses, depending on the the DM's chosen rules:

• If obeying the first rules described in the FAQ the creature has Base Attack Bonus of +14 if class levels are taken in the most advantageous way.
• If obeying the second rules described in the FAQ the creature has Base Attack Bonus of +17 if class levels are taken in the most advantageous way.
• If obeying the rules alluded to in the Monster Manual, using the frost giant jarl as an example, the creature has a Base Attack Bonus of +21.

(By the way, don't think that a Base Attack Bonus of higher than +20 is impossible or anything. Despite the existence of Table 6-18, creatures in the Monster Manual from their Hit Dice routinely have Base Attack Bonuses greater than 20 (e.g. big dragons, the solar, the tarrasque).)

"Isn't That Overpowered?"

When is a level 30 character not overpowered? I know, I know: When he's a monk. But, seriously, though, this creature really isn't. A naztharune rakshasa Rogue 8/Scout 6 has no ability to cast spells, and besides its DR and SR, the special abilities it does have could've been gained earlier via judicious selection of other classes. What it does have are good ability score adjustments, but to get those required throwing away 5 character levels. Beyond its laughable SR 21 it has no defenses--except for its 4.3 million gp--against even basic magical attacks. It does have sneak attack +17d6, which is something, but it has a mere 9 feats, only 2 of which can be epic feats. I'm certain even a Brd30 or Ftr4/Rog26 (who'd have only sneak attack +13d6) would fare better in epic play.

But, hey, that picture's pretty cool.