Staring 1.30 theme update you can hide the guest number easily.

Hide guest from the booking form

Put “What do you want to rent” to Object Rental

You will see the booking form like this

And the submit form will exclude the guest number

Hide from contact owner form

This is done with css

/*remove guest from owner contact form*/
.modal-body #booking_guest_no {
display: none;

Help for custom css.