Can´t give you screenshots of a function non-existent, but I´ll do my upmost to describe it accuratly

Linked Acccounts is a helpful tool for providing users the option to add a second, connected account for different use cases. But in most use cases a user only needs one or two linked accounts - and you as a webmaster don´t want users to have unlimited linked accounts, since this 1. will undoubtly be abused, 2. leads to greater confusion for visitors and members, when linked accounts of a single user interact with each other in discussions, 3. supports starting flamewars, trolling etc and adds a huge load to the moderation work since you first have to figure out who is who to recognize that this is a trolling in the first place. So, in fact, I can´t imagine a use case to allow for unlimited linked accounts at all. On the contrary, unlimited linked accounts would make me not use the add-on at all on live forums. But that´s my point of view regarding linked accounts in general.
Regarding limiting the number of linked accounts, the idea behind is quite simple. If you have to use paid usergroups with subscriptions to monitize your forums because you don´t want to have ads in it, Linked Accounts shouldn´t be available for everyone. Instead - as in my case - I want to have
- no Linked Accounts for the free membership
- [x] Linked Accounts for the paid usergroup #1
- [y] Linked Accounts for the paid usergroup #2
- [z] Linked Accounts for the paid usergroup #3
- aso..
That way it adds to making the subscriptions more attractive.