Daves info is spot on but I'll add some. You can check out
ccdcalc.exe to find how various scopes and CCD combo's go together and part of the info given is Critical Focus Zone (CFZ) per combo.
As Dave said shorter FL scope have smaller CFZ...so you want lots of steps from your focuser in this zone...not too many as it will be verrrrry slow! Longer FL scopes...not so critical.
You can calculate various stepper combos on your focuser by Circumference
C=πD (lets use D=5mm, the pinion for the drawtube diameter, for arbitrary sake, then each revolution of the pinion = 15.7mm movement). If I choose a stepper that has 2040 steps per revolution (one I used, has internal gearing) then I get each 2040C=π5 ~ .007699mm (~7.7 micron per step, 15.7/2040).
In a scope that needs CFZ = 20 microns then this is not good I'll need a stepper with more steps per revolution either thru smaller step angle or bigger gearing...or even a smaller DT pinion.
If you connect via a 10:1 reduction pinion drive then obviously you will get 10 times more step accuracy per step "point 7 microns" per step (depends on the pinion). This is probably just acceptable for this small CFZ. Given lets talk an F7.5 scope as an average scope then it's CFZ is ~123 microns so 7 microns per step is fine.
Anyway hope this helps, try not to choose a stepper that has too many steps per revolution as movement is slow...but more accurate.... Have a look around this may help you decide on choosing. No one can tell you how many steps you need in the CFZ...the more the better but too many and the focus movement rate is very very slow!
hope this helps.