Humidity sensor switch wired to current setup
Those white wires are critically important

What you have there is a classical power-to-the-switch-box setup, with a power feed going in, some switches, and both switched-hot and always-hot feeds going off to other places.  As a result, that bundle of white wires in back is your neutral bundle, and is thus critically important to you since your humidity sensor needs neutral so it can power its own innards without trickling power through the load.

Wiring this will require a couple of wirenuts, by the way.  With the breaker off:


Unmount the old switches but do not unwire anything yet
Remove the bottom wire from the switch you are replacing; this will be your always-hot feed
Cut the wire in two and strip both cut ends back a bit further
Wirenut both those wires to the black pigtail coming out of the new humidity-switch
Remove the top wire from the switch you are replacing; this is your switched-hot for the fan
Wirenut that wire to the red pigtail coming out of the new humidity-switch
Fish the bundle of white wires out of the back of the box and nut the white neutral pigtail from the switch into it -- it's important that all of the white wires get nutted together here, as they are the return path for power to get back to its source!
Fish the bundle of bare wires out of the back of the box and nut the green grounding pigtail from the switch into that bundle.  Once again, it's important that all the grounding wires get nutted together here
Carefully stuff the bundles of white and bare wires back into the back of the box
Mount the new switch in the location vacated by the old
Mount the remaining switches back into their existing locations
Button the box back up, turn the breaker back on, and enjoy your new humidity sensor