A few moments.

Each app might have an own license which assumes different conditions. To learn the definite module license: open it > look at the right column 'License' (may reference the tab 'License').

Different licenses assume different terms.

Default license for paid apps is Odoo Proprietary License v1.0. It let you use the module as library: so, you might inherit its features or use them as they are in your projects

Often it also is assumed that one copy of the app is used for a single production database. So, for various projects it seems that you should buy the app a few times.

For details it is better to contact the Odoo store representatives: https://www.odoo.com/contactus

Note: I'm not a legal adviser, so my understanding of license terms might be not fully correct.