Gee, teenagers these days must have a lot more friends than what i did in school! i had maybe 15....? and thats over estimating.
I think 40 is too many to have a party at your house. 20 is reasonable but again pushing it. I think you should really have some ground rules. If you trust your daughter to behave herself and she has good judgement in her friends then go ahead with it.
As for posting it all over FB you have the potential to have gate crashers and the whole thing could be a complete disaster. I think you should tell her to remove the event off her FB and text/call the people she wants to come and leave it as that.
When i was her age i went to a few parties that got SO out of hand was supposed to be not many people and turned into hundreds.
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