As a wholesaler, it’s often best to not have a monopoly on a wine list. Majority, yes. Monopoly, no. There is a draw that the owner/buyer at the restaurant will feel toward to question of “I wonder what working with other companies is like?”
There is no magic number, for it’s based on the size and needs of the restaurant, but quite often having two or three vendors/suppliers with pretty equal footing on the list makes for healthy competition and efficient buying and meetings.
If as a wholesaler you do have a monopoly, or are offered one, have the guts to say you don’t want it. But also be very clear in who you’d suggest also gets some business. Have a short list of the good reps from the competition that get it, that aren’t land grabbers, that aren’t only in it for themselves.
“They’ll do you right” is what you can tell the restaurant buyer, and you can be confident you’re not destroying your own future.