For exact numbers you would probably have to check with someone at the shops. To my knowledge and working from a 2005 list I counted:
22 Reserve Engine Companies
2 @ Bureau of Training
45 dedicated spares
3 93/94 High Pressure spares
1 93/94 that has been rebuilt into a rescue pumper and serves as the reserve squad
1 92/93 that has been rebuilt into a rescue pumper and serves as a spare squad piece
For a total of 74. My guess a good number of these have already been permanently removed, and even more will kick the proverbial bucket when the new deliveries finally arrive.