you can normally get away with about 10% empty seats for 5%
I've had 19000 for a 20000 stadium and still got 5%
Although it's always worth just dropping the price to fill the stadium, 5% possession is worth more than a few pounds/ euros/ dollars in the bank...just make sure at the start that your car park is ok or you get a message saying fans couldn't get in because the car park was full!