It depends on what sort of ceremony you're having, what time of year, and where it's going to be. Eg you may have less guests confirming if you're only having a short do, or if it's during the holiday season and everyone's booked their holidays already, or if you're holding it on a weekday, or if you're having a destination wedding.
But if you're having a standard wedding, I would guess at assuming 10% will decline. But you can't send out invitations with that at the back of your mind. You have to be able to cater for all of them, in case they all agree.
We are sending out invitations to a couple we know, and for a reason I won't go into, we are 99% sure they will decline, because although they want to come more than anything, they can't. But even though we're expecting them to say no, we're holding their seats for them until it's confirmed.