Consider the fact that some of your TN licenses will be consumed by trunking, rans, music, etc. Those won't be part of OTM station admin and so no need to pay for that. You do need sufficient licenses to cover anything configured as a phoneset (real or virtual), both existing and potential. On smaller PBXs, it is common to order the same number of OTM licenses as PBX TN licenses. But on a system with 2000 TNs, that would be pretty expensive.
It is more common that not enough RU's are ordered. (RU stands for Reporting units and is the basis for licensing the Billing applications in OTM. A reporting unit represents a single entity in the OTM Corporate databases to which costs/usage can be assigned and reported on through the Billing applications. An entity can be either an employee, an external party, or a role or project in the Employee, External Parties, or Roles/Projects databases respectively.) How many of those do you have?