I guess I may be wrong but I don't think it's that critical
I had sever guitars sitting in both dry and humid conditions and all I needed to to was a minor tune up on the strings.
You can buy a humidifer and a dehumidifer but thhe one I fixed up has a stat on it and it's 65% humidity here but in the other room it's cool and 40% humidity/
Check out some small portable units on Amazon
BOB LECH AF2Q
Former member of the 12:00 Oclock Skydiving Team