You can send up to 500 invites per day. We try to strike a balance between providing artists with enough invites to give everyone on their press list an early listen, and not opening up the possibility that a less upstanding individual could use the service for spamming (thereby ruining the party for all). If you’re looking to send to a larger list, you might consider download codes instead, which also work with private tracks and albums and have no such restrictions (since you, not Bandcamp, mail them out).