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.