I get 10–20 new subscribers per day on a regular basis. This is pretty good since I have a lot of repeat readers, so it’s not always easy to grab new ones.
If you aren’t getting many subs from your call to action, it’s probably the message or the offer or both. I haven’t seen yours, so I’m spit-balling here, but the CTA needs to be a benefit for the reader only. Oh, and never ask people to join your newsletter. No one wants another newsletter.