You can only have one title at a time. When you take the other one, you will lose your current one, and actually free one of the slots for it.

So I dont see ANY information on these really anywhere

The titles from Amaterasu.

First off they all seem to be available in limited quantities like "first come first serve"
example: current successors 7/200

If that is so then that kind of sucks

Also curious if they can be overwritten? It definitely seems like some are fairly weak while some ridiculously OP and looks like you only get room for so many?

This would also suck if not as I already took 1 that was +4 vit +200 hp and now I see another that is +8 vit +400hp

Can anyone shed some light on these or please explain anything they know about them ty.

Once you buy a new title it overwrites your old one yes. They don't stack either.