Well, for the bioload it really does depend on what apple snail you have. If it’s just a mystery two should be fine, particularly if you have some plants to help with the
nitrates. But if it’s a regular apple snail I don’t know if I’d two or even one for that matter. They have big bioloads and get large, I can see them comfortably staying in a 10 gallon with a betta. It seems like it would be very cramped... I could also see the bioload being a major issue as they mature.
I do suggest making a thread and seeing what kind of snail it is, this way we can properly answer your question on the bioload.
As for waterchanges, a good place to start is between 25-50% weekly, but as you potentially have a very large bioload it’s likely better to do no less than 50% weekly, perhaps 75% instead.
If you get a
liquid test kit(which I highly suggest) then you’ll be able to monitor how your water works and go from there.
Sometimes you don’t really need to do a waterchange every week and the parameters will not get all messy. Take my planted 10 gallon for example, it’s fine to just do small waterchanges weekly or take a week off.
I know some of the things were relatively answered, but wanted to give my input as well.