MacBook Pro: necessary specs for virtualization
Depends on specs and activity of the boxes. 15x one core and 512MB RAM will run on a Core i5 and 16GB RAM. If the boxes are going to need more RAM, you'll need to run fewer boxes at a time. A VM typically requires about 100MB RAM on the hypervisor side, macOS and any tools you might have running also need some memory, so those are the constraints of the system you'd be working with. More RAM isn't an option, it's soldered on, and 16GB is the limit.