Okay so, I see how to make a counter using expression controls but it looks like may be limited to fairly small numbers. The project I'm working on right now requires 21,000,000,000,000 with the commas. I'm not sure how exactly to get this size of number to count up using methods that I've seen. My client is really set on doing it with rolling odometer style counter so I'm pretty screwed at this point. Hope someone out there has some idea or experience with this.
Use the "Advanced Odometer Demo - Angle" comp as your starting point. It uses an Angle controller instead of a Slider controller to allow huge numbers. Ignore revolutions for now and just type "21000000000" as the number of degrees. Duplicate and arrange the "Digit Counter" layer as many times as necessary. BYOC (bring your own commas).