These people may be able to answer your question. They are the UK largest supplier of RRV's
https://www.txmplant.co.uk/
Yes there are hundreds of RRVs and with their associated trailers owned and operated by Network Rail's Contractors and sub Contractors.
Over the years since their introduction in the late 1970's they have replaced Locomotives and wagons
for use in engineers possessions in transporting materials to, from and within possessions and not always in a safer or more efficient way.
Infrastructure Engineers trains are mostly still used for transporting long welded rails and new ballast to sites and for removal from site of spoil/ spent ballast and in certain circumstances old track.
But generally other materials are moved by heavy lorries to the nearest road/rail access point, thence to site and vice versa, by RRVs with trailers. This often increases the number of possessions/ closure of lines to scheduled services and takes very much longer than it might take using an Engineers train.
There have been numerous safety related incidents over the years causing deaths and injuries to track and associated workers. One of the most tragic that comes to mind was when some years ago an unbraked RRV trailer ran out of control and killed trackworkers near Tebay on the West Cast main line. Lessons have regretfully been learnt the hard way with RRVs.