I'm using currently 4:

- One pi4 for my homemade 3D printer (a Voron)

- One pi3 for a homemade eInk clock/weather station (here I could've gotten with sth much cheaper like an ESP32, but the Pi is so convenient)

- One pi zerow 2 for a homemade eInk radio

- One pi4 I just bought for home automation (homeAssisstant)