oh quick google for my area comes up with ..
1,000 vehicles on both emergency and non-emergency operations.

bit much for the garage down the road, and the cost of it, might be wiser for the trust to employ their own
The North West Ambulance Service (NWAS) NHS Trust was established on 1 July 2006, by the merger of ambulance trusts from Greater Manchester, Cheshire and Merseyside, Cumbria and Lancashire.
...
The Trust operates around 1,000 vehicles on both emergency and non-emergency operations.
http://www.nwas.nhs.uk/about-us/who-we-are/