How to find out how many sensor you can add to a I2C or SPI protocol?
I2C will allow you to add as many as there are unique addresses available for the device dependent on line conditions. If you need more addresses than available then you will need to use a I2C hub or multiple I2C buses.

SPI allows as many devices as you can provide nSS lines for, again based on line conditions. A multiplexer such as the '138 can help here.

Regardless of which you pick, the more devices you have the slower you may have to run the clock in order to allow enough slew on the clock and data lines.