"Oh thank you so much for the offer! I need to get a little further along before I can start even thinking about showers, but touch base w/ me in a couple months. Again- thank you so much!".
If you get multiple offers and too many for it to make sense to have that many showers, then you start saying "Oh, thank you, but I actually already have __ showers planned." and if you think one of your existing hosts wouldn't mind, you can say "if you want, you can contact ___ to see if they need any help".