Partners should refer to the ISE Ordering Guide for details.
But to confirm your specific question, the part number for ISE Base licenses is constructed thus:
Cisco ISE Base License: L-ISE-BSE-xxxx=
(xxxx = 100, 250, 500, 1K, 1500, 2500, 3500, 5K, 10K, 25K, 250K) endpoints
How many endpoints to order is based on the anticipated number of simultaneous sessions. A Base license is consumed when an endpoint establishes an active network session (RADIUS accounting "Start" message). The license is released when the session ends (RADIUS "Stop").