Buy subscription gives for all the new projects you create or just one.

If I buy a subscription, does that give me for every project I create or just one?

When you subscribe to a plan, that plan is applied to the team folder you are upgrading. That team folder can contain multiple apps.

As for the limits of your plan, some are per app, such as row limits…but others are per team, such as user limits, update counts, and storage limits. Teams folders will allow a certain amount of apps to be published, or a certain amount to be whitelabled.

It’s best to study the pricing page to understand what each plan offers.