Anyone having an uninvited guest table?

We didn't have an extra table and we also didn't have one single uninvited guest show up.

Nope! I think that is totally unnecessary. We had one couple (one of our groomsmen and his wife) that brought their toddler even though we had an adult's only reception. The groomsman had asked about bringing the child to the rehearsal which we said was fine so he misunderstood and thought he could bring the child to both the rehearsal and the wedding. They live like an hour from the venue so we didn't think they were going to stay the night before the wedding, but they decided to. I wasn't thrilled that they bought the child, but adding a high chair for her wasn't a big deal. They also had to remove a chair at a different table because we had forgotten to since my husband's cousin was in a wheelchair. We told them the night before when we realized the child would be at the wedding and when I realized we didn't need a chair for the cousin. Otherwise, we had no other guests show up or any problems like that. If we had, there wouldn't have been a place for the uninvited guest to sit or food for them to eat, but that would have been their own fault for showing up uninvited. You shouldn't have to pay extra for those who are rude enough to show up uninvited.

Most venues know how to deal with this and will make it smooth if an uninvited guest shows up. What is more likely is a RSVP decline to show up rather than a completely uninvited guest. The venue should make room for them and will ensure no attention is drawn to that person. The table would be humiliating for them. I don't expect this is a common issue but I'm sure once in awhile someone forgets to RSVP or accidentally hits decline on your wedding website and then still comes.

I have seen this advice, but knowing how much it is per head the answer will be a NO.

That sounds like a waste of money and space. I mean it sounds like a good idea, but you would have to pay for those seats, right? Our venue has a per head price and they are basically putting out the number of chairs based on our headcount. If someone declined or wasn't invited, they have no reason showing up. Also, I've been to many weddings in the past, where people RSVP'd yes and did not show up, so those were extra seats. If people do not follow your rule then they will have to share a seat with their child or leave.