The invite count is determined by Facebook. We tried our best to explain it in the description for the "Max Invites" parameter in the module, so I'll quote it again here:
The maximum number of users that can be selected. This value ranges from 1 to 35, and is capped at the number of friend requests the user has remaining under their limit. This attribute is ignored if it is greater than the number of requests your application is able to send (which is determined by Facebook).
So basically, you can set it to a max of 35 (1000 would be ignored by Facebook). However, Facebook ultimately decides how many invitations your site is allowed based on the amount of users your application has, amount of invitations sent by your users in previous sessions, and whether it considers previous invitations to be spam.
Maximum invites for JFBInvite
