TfL have published their annual bus fleet audit
Data at TfL year end 31st March 2023
Bus fleet 8643 buses, down 152 from 8795 year earlier
The preamble says 949 electric, 20 Fuel cell, 1 new routemaster retrofitted to electric, 997 hybrid new routemasters (so 2 must have been withdrawn). The 2 single deck fuel cell buses which hardly ever got used also gone.
RATP has youngest fleet averaging 6.21 years, Arriva has oldest fleet (except small Sullivan) average 8.99 years
Just 201 new buses entered service in 2022, which I think is lowest for at least 30 years
EDIT
just checked March 2017 report and there were 9616 buses, so fleet has shrunk nearly 1000 buses