You can add as many numbers as you want. Each WhatsApp number corresponds to an instance of Wsappmass (exactly how it works for WhatsApp Web).
If you want to activate multiple numbers, you will need to activate multiple instances. You can also log out and return to Wsappmass with another number.
The important thing is to remember that each instance of Wsappmass has a WhatsApp number, but there are no limits to the accounts you can create in Wsappmass.
And remember that thanks to Wsappmass, you can use one WhatsApp number for multiple devices.