depending on the track and your batteries, anything from 22/75 to 26/78 - I've run both and in between these and had good performance regardless of track.
If it's a medium sized outside Astro track, give 24/78 a whirl for starters. If it's tight indoor carpet, try 22/75. Roughly speaking, 3 teeth on the spur equals 1 on the pinion, so adjust accordingly.
HTH
