Republican Legacy

[Trin Tragula]

There is still no limit on how many Republican vassals you can have. The limit is for how many counts you can turn into republican vassals using the interaction

EDIT: Also worth knowing is that this tradition has some pretty specific prerequisites. Your culture has to be related to Roman culture at some point in its history. This means you may have to hybridize with an Italian culture to be able to pick it.

 

[Trin Tragula]

is the ai restricted from instantly game overing the player using this interaction. the dev diary gave no indication one way or the other

It won't happen to the player
There are also a bunch of of other standard restrictions:
AIs will also refuse this if they are heirs to titles, have landed relatives, or have more than one county to their name.
The county itself must be of the culture that has the tradition.