A successful party for young children does not necessarily depend on elaborate decorations or favors. Success comes from being prepared, involving the children and sticking to a central theme.
The question here is not so much how many you can physically handle but rather how confusing the party will be for the children. A good rule of thumb to follow is to invite no more children than the age of your birthday child plus two. Extra children could be invited if they were used to playing with one another.
Actually you are probably better off without mothers at parties after your child reaches three years of age, except for one good helper. If other mothers do want to come, discourage them from just sitting around talking and not participating in the party. Put extra mothers to work by giving them a specific job, encourage them to join in the action.
Don’t forget Dads. A weekend party for just kids and dads can really be a big hit with all involved. Children have a tendency to whine less with Dads in charge of the activities.

Be prepared with some activity to entertain those who arrive first. Perhaps decorating a paper sack in which they can take home their party favors. I like the ones with handles best.
I have found it best to delay opening presents until all children have had an opportunity to win or get a prize or party favor of their own.
It has been my observation that children enjoy a party more if they have been allowed to help in the preparation in some way. Plan an activity in which they can participate. Such as; making table decorations, making their own favors, decorating the dessert, churning the ice cream, etc.
Plan most of your games and activities so that all the children can participate and be a winner. I like open-ended games, where everyone plays on their own level and everyone wins.
Take time to explain what is going to happen next at your party to the children.
Think simple! Have as much food preparation done ahead of time, if possible. (Unless it is a planned activity to be done with the childen)
Cupcakes are easy for children to eat, especially the kind that are baked in ice cream cones
Ice cream can be cut or scooped in advance and kept in the freezer. Bon Bons are even easier for young children to handle.
Losing favors can spoil an otherwise enjoyable experience for a young child. The conscientious hostess will provide some suitable container like a paper sack for each guest with his name on it. Encourage adults and children to return lost favors to the correct child’s sack. Also, be sure to buy or make extras of all favors in case of accidents and lost items.
Party decorations for a children’s party can usually be confined to table decorations.
Be sure all balloons are big and easy to blow up, plus they should be hung up high out of the reach of the children.
Paper products. - I usually went with plain plates and table cloths, but used theme napkins and cups.
As a rule, activities generally take less time than planned. Always have a few extra games or activities to fall back on if the children finish quickly or seem uninterested in one of the main activities you have planned.
Don’t make children wait while yu clean up after an activity unless you allow them to help. This is the time a helper is most valuable. They can merely take the children and start another activity or else clean up after you start a new activity with the children.
No preschool child can enjoy a party unless his basic needs are met.
Plan parties so they do not interfere with nap times.
Eat when the children are hungry. And don’t expect children to eat much ice cream or cake if they have just filled them up on lunch or candies.
Plan a potty break, or have your helper take the children to the restroom through out the party.
End your party near the time you have set.
Give the children something to do while they wait for their parents to get them. (Such as, blowing bubbles outside.)

Be prepared, relax and keep things simple!