The current deployments of the Siege Engineer Corps puts about 80% of them in the tunnel reinforcing it for permanent use.

There are about ten percent of them in the armies rearguard - they aren't really in demand for this assault, and are more likely to get deployed with their weapons on the walls once the city is taken. I don't fancy figuring an exact number of men, but I'd say there were about 200 well supplied siege engines with their crews backing the army right now, though these have not been deployed into combat.