Highly probably it will use I2C connection. When 2 ways can suggest.
First is to use sensors with possibility to change I2C address, but not wide choice actually of them.
Other is to use I2C multiplexer, like TCA9548A - will allow to connect more sensors with same address to virtual I2C buses.
The optimal solution would be something that looks like Dallas components temperature sensors, which several sensors easily can be installed on one board.
Surely there must exist some similar hardware, that measures humidity instead of temperature…?
There are many sensors that do humidity (and temp for free). As mentioned above they use i2c and wiring length is dependent on how many and type of wiring. DHT11 and SHT30 etc. They need to be calibrated and position is important.