It's hard to say. I depends on the weather conditions, how much sun your pool gets, how big of a swimmer load you have, etc. I'd probably go ahead and buy 10 of the big jugs for now. You'll end up using it.

Most of us use generic Walmart bleach. By the way, not all bleach is the same concentration. Some places like dollar stores sell 3% bleach which is typically not a good buy. You may also find a pool store that sells liquid chlorine that is usually either 10 or 12.5% sodium hypochlorite which is the same ingredient as bleach. If it is at a good price, you can use that and it will save you from having to buy so many jugs of bleach since you have a pretty big pool.

You may be able to find CYA at Walmart. Just read the ingredients. You don't want a mystery mix.