From PHB, p. 203:
Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in chapter 5) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.
In other words, if no cost is listed, then your pouch/focus should be enough. Many of the trivial components are meant for flavour/fluff/comedic effect (see: those that require guano; or the lightning spell that require fur + glass, to generate static electricity).
Also notice that Bless requires a "sprinkling of holy water," which seems to be a trivial amount, unlike like a flask.
What about the Commune spell, which explicitly lists a "vial" of holy water?
The same reasoning applies (no cost listed, so, focus/pouch takes care of it), though granted, it's where the rules start to keel under the pressure of realism. A GM would obv. not allow a player to have such a profitable activity, and he needn't prevent it by fiat: he could just reason "if that were possible, every Cleric/Pally could do it, flooding the market and erasing all profits" (or something along those lines). But again, this is just catering to realist intuitions, which can become a slippery slope very quickly.
Your spell component pouch counts here if the DM agrees
The description for a spell component pouch says, "...has compartments to hold all the material components and other special items you need to cast your spells, except for those components that have a specified cost..." (PHB. 151). The description does not say anything about the pouch being magically filled or having an endless stock so it's really just a conveniently designed pouch for holding things your character gathers in the background.
You regularly cast Bless which has the material component, "a sprinkling of holy water", (PHB 219) so that's available in your pouch. The pouch description doesn't say you've got an infinite amount of it so this isn't some loophole to get free Holy Water flasks, it's just a point of convenience.
Flameskulls, "...reform unless they are splashed with holy water...", (MM. 134). Now strictly speaking you need a splash of holy water and your pouch contains some undefined number of sprinklings. Fair rulings would include that you don't have enough holy water (bit boring), that you've got just enough and now have none left to cast bless (add tension), or that a sprinkle is good as a splash and inspiration for your troubles (reward thinking about your resources).
You'll need to talk to your DM to figure out how this will work at your table.
Best Answer
If they didn’t bother to print a number of usages, it indicates that the amount consumed that way is, in their opinion, negligible and not worth worrying about. Having a flask of holy water be consumed by repeated castings of bless would be a houserule, and something that should be covered at the game’s outset, before anyone has so much as decided to play a cleric (much less buy holy water or prepare bless).
I would strongly caution that this houserule is unlikely to add much to your game. It just becomes another thing to track, and there just isn’t really much of anything to be gained by the additional bookkeeping—which is probably why Wizards of the Coast didn’t bother.
This logic, of course, makes one wonder why we bother with (zero-cost) consumed spell components at all—which is a very good question that is well worth considering, and possibly doing something about, e.g. removing them from the game entirely. There is exceedingly little value here.