Access bus vehicles have low floors and an internal layout that provides more space for wheelchairs and other mobility equipment than a standard bus.
Rachel Jones, head of mobility services, said: “It is a very popular service, and is constantly close to capacity.
“The existing fleet is well beyond its operational life, with more issues with the vehicles that are having an impact on the services we can provide.
“We need to review the fleet in its entirety.”
Only a handful of the buses will be electric, with many being low emission diesel, according to the Local Democracy Reporting Service.
A report to members said a fully electric fleet would prove too costly, adding: “The aim is to trial the effectiveness of electric buses in operation, with a view to future vehicle replacements being fully electric.”