UBB.threads PHP Forum Software Community Forums UBB.threads 7 Version 7 Support RSS Feeds options question

1. I know virtually nothing about RSS Feeds.
2. I am therefore assuming that users will subscribe to them and an e-mail will be sent out.
3. Again, assuming these RSS Feeds are sent via e-mail, what is "expand it a lot"?... 1000, 1500, 3000, etc.

Thanks for responding.