Maximum number of invites to a Facebook event
From the Facebook help pages:
You can invite an unlimited number of people to events, but you can only invite 100 people at a time. Once you invite the first 100 people, you can then start inviting more. In addition, you can only have 300 pending invites at one time for an event. Some people will have to respond to your event invite before you will be able to add more.
I wish it had told me so, rather than let me click away, only to realise afterwards that some of the invites didn’t get sent…