Is there a limit to the number of manga you can be following?

[Chronigan]

Or will the site automatically remove things from your list if you haven't read them in a long time?

 

[Angry_Panda]

To the best of my knowledge (note I'm not staff), there is no 'intentional' limit - if you tried to add every title on the site to your library (please don't do this), the site won't attempt to stop you. It's possible you could run into some odd back-end limitation once your library gets big enough; whether that happens would be a dev-type question, and I'm not one of those. But I know there are members here with thousands, if not tens of thousands, of titles in their library.

No, the site won't 'remove' anything from your follows for inactivity, either you not reading or no uploads occurring, though if a title entry was removed (very rare but not unheard of), it would disappear.