If you have 2 ISR routers, one acting as Failover, do we need to have both the same number of SRST licenses on the 2 routers?
No. You will only need the SRST licenses on the primary router. Because this feature provide call-handling support for Cisco Unified IPphones if they lose connection to remote primary, secondary, or tertiary Cisco Unified Communications Manager installations or if the WAN connection is down, and Cisco Unified Communications Manager users will gain access to more features during fallback without any additional licensing costs.
more information about fallback state you can check this link below: