The main factor to determine this will be whether you are having the party at home or in a hall. However other factors will be your child’s wishes and even your child’s age!
Obviously if you are having the party in hall, you can invite as many children (within reason) as you wish. However I really wouldn’t recommend you invite too many (IMO 40 is too many!). As a professional entertainer I have a powerful amplifier and wireless microphone. Assuming you haven’t got this equipment you will find it difficult to be heard by 40 or 50 loud and excited children.
When the birthday child is 5 or 6 years old, they will often want to invite the whole class. This will mean 25 to 30 children. In a hall, I would say this is the maximum you’ll want. Problems arise when siblings are invited or the party is a joint party, which could easily push the total number of chilren over 40 (or even 50). Don’t do it! You’ll have to be firm with your child or your friends and family! 🙂
As children get older you’ll find they are more selective as to who they invite. I’ve found 8 year olds (and older) will only invite their closest friends and will often only invite friends of the same sex. This makes everything more managable.
In your house of course everything is different. Even if you have a large house you’ll seriously have to cut down on the number of children your 5 or 6 year old will want to invite. You could have the party in the garden, but as we all know the English weather is so unprecictable. Many times I’ve been asked to do the party outside (in the middle of summer) only to pick everything up 30 minutes later when the weather turned. This is something you really don’t want to do.
If you slide the coffee table to one side and rearrange the rest of the funiture I think 10 to 15 is a comfortable number to have in your house.
Of course small parties can have their own problems. I have performed at a children’s party with just 3 children! This makes it difficult to play games and get the children involved. Also I happen to do balloon modelling so I believe each child at this party went home with 10 balloons each! Invite a reasonable number (there will always be no-shows) and everything will go well.
Finally, make sure the birthday child actually gives out the invites! I party I did several years ago and the birthday boy forgot to hand out the invites and wondered why nobody turned up! This was a tough party!