I'm always on the lookout for information like this as well. According to Davide Pastore (if I counted correctly) both sides had about 55 guns (all types) in the vicinity: 41 in the engaged portion of McDowell's force and 14 at Centreville; 20 with Johnston's Army of the Shenandoah and 31 engaged of 35 pieces listed for Beauregard's Army of the Potomac. Of course these numbers may not be exact. A tedious but ultimately rewarding slog through the Official Records would be the next step:
Thanks! I'll look for them. If anyone has the approximate brigade strengths within the divisions (which numbers I have) I would greatly appreciate that.