Thinking back to my time in HQ 4 CIBG in the mid-sixties, we had something like 75 Centurions in Europe. The armoured regiment had three squadrons plus the ones in RHQ and the rest (about a squadrons worth) were in storage in a Brit Depot to be fed into the Canadian component of the British armoured delivery system in wartime. (I was responsible for keeping an eye on operational equipment, both issued and in the replacement system, and monitored the upgrading of the Centurions to Mk XV or something akin.)
With the move south, they were moved somewhere, and were available along with the ones made surplus by the regiment's downsizing for issue in time of crisis. Leaping ahead to the late-80s, FMC came up with a plan for a division in Europe including two armoured regiments, each of about 32 Leopards using the tanks for the 4 CMBG regiment and the replacement ones in reserve.
Sorry I can't be more specific, but that's how I recall at least part of the issue.