This is always a tricky one to balance.
Our colleagues have been to a quiz where teams were limited to three, with no exceptions. This did mean that couples who happened to be in the pub could join in, without thinking they were going to be at a disadvantage. However, three is an odd number (in both senses) because it prevents two couples, or mum, dad and the two children, from joining in.
At the other extreme we have seen a team of ten at one quiz. There were several “less than friendly” looks and comments about this from other, more normally sized teams . . . especially when they won!
The best approach is to advertise the team size limit in advance, and four or six are a good limits. In this way a team of ten can’t claim to be unaware, and can’t complain too much when they’re split into two teams. Although on a related note, you should ensure they don’t mark each other’s answers!
Equally, you could suggest to two or three teams of two that they combine forces, especially if you are running out of tables.