In all Pokemon versions, there's a trade off: You can easily level a few Pokemon off of trainers, or slowly grind to level your entire team.
With the goals of beating the game and catching as many possible, what's the best strategy for efficiency in terms of the number of Pokemon you train throughout the game?
What about HM slaves?
For example, should one only focus on three pokemon from the beginning to end game?
Best Answer
I have never thought of the Elite 4 as "End Game," just as I never though of hitting level 60 (and later 70, 80, 85) as "End Game" in WoW. Rather it was just another stepping stone. Because of this attitude, I optimized very differently.
First, I found that 3 Pokemon was about the sweet spot for training at a time. Usually this entailed 1 or 2 (xp share, or swapping) I was focusing on, and a third for backup (someone beefy enough to roll whatever area I was in).
Next, I specifically targeted Pokemon whose EVs I could easily raise. There are a couple areas where you can guarantee which EV you'll get, but usually I lead with a Pokemon whose EVs I didn't care about, and then swapped according to what I ran into (in the case of trainers I could set this up more intelligently). As a result, I was usually targeting only 1 or 2 for actual EV leveling. This process would recur every time I got a Pokemon whom I wanted to level (I usually decide these at game start).
Once a Pokemon had been EV leveled, I could then use it as a clean up for other Pokemon's EV leveling. From a pure XP perspective, having a "clean up" Pokemon (sometimes initiator) means that that Pokemon will be receiving more than its fair share of experience. The end result is that usually the Pokemon that I EV train first end up being my strongest (not just because of EVs).
Finally, as I originally stated my end goal is not the Elite Four; rather I'm eventually targeting competitive play. However, EV training other Pokemon doesn't max out my first couple, so ultimately I end up doing runs on the Elite Four to power level them fast. Usually, by the time I hit my first run I have 4-5 beefy Pokemon fully EV trained, so it's not a major challenge. However, sometimes I haven't spent enough time and that usually means I need to take the time to EV train up a couple more Pokemon so they hit that sweet spot where they start firing on all cylinders.
I realize this may not be exactly your use case, but I think it is something to keep in mind.