I've seen a startup in the Netherlands that never made it beyond mega-client #1. They are still doing well, but basically they have become a service provider for what was their first bigger customer and, years later, is now their only one.
This is in line what others here are warning, but the way I would deal with it is specify a price where either it is similar to the single-seat price with no longer term obligations OR a much reduced price with a very long contract duration (so they can either make you so rich that you don't need them or they can save money but then they cannot threaten to leave you as a means to elbow you into building off-roadmap bespoke features that only they need). Note that this is theory ;-) I have not been in that situation myself.
This is in line what others here are warning, but the way I would deal with it is specify a price where either it is similar to the single-seat price with no longer term obligations OR a much reduced price with a very long contract duration (so they can either make you so rich that you don't need them or they can save money but then they cannot threaten to leave you as a means to elbow you into building off-roadmap bespoke features that only they need). Note that this is theory ;-) I have not been in that situation myself.