From what I know, Yes, every store membership is individually charged.
Here's more info on different membership plans
Welcome to the Shopify Community. As @Nick_Marketing mentioned, each store must be on it's own paid plan. As nick_marketing also shared, the plan details and pricing can be found here. Do you have any questions about what each plan offers?
To clarify further, there is no limit to the number of products you can sell on each store. Each product can have up to 100 variants. This is the same for all plans.
Do you have multiple businesses you are looking at bringing to Shopify? We have great resources on an introduction to Shopify, as well as migrating to Shopify from another platform. Let us know if we can help answer any questions!