I have about as many referral trackers as you do. And I have the same results where most were never used. And a few have tons of signups (but most never wrote hubs).
I started out making a tracker for each referral. But I later found out that this is not necessary since you can click on the tracker name in your list of Referral Trackers (in the Earnings page) and it expands into a list of all the Hubbers who signed up under that referral.
I don't know if there is a limit, but based on what I just explained you might agree that we don't need to have so many anyway. Maybe just a few so we have a reference to the sites we displayed a referral link on.
As for your question about deleting, I never found any option to remove referral trackers that I had created. I wish we could, so we can clean up. And it would be nice to be able to move signups from one tracker to another. But we can't do that either at this time.