It depends on the mod and the build. For the sake of argument in this explanation, we're gonna assume a single cell Mech Mod. A 20A CDR cell can handle a build down to 0.2 ohms safely, whereas a 30A CDR can handle down to 0.15 ohms safely. Ohms Law is in play here, so the build has to be regulated to the battery. Regulated Mods essentially manage themselves, and won't really allow you to over-stress your batteries, If a build is too low, it won't fire, and if the batteries can't handle the load, the mod shouldn't fire either.
In general, most people are more than happy with 20A CDR (which are the standard) for their regulated mods, because on average, most people don't push their mods past 120W, but more advanced builders/mech users like 30A CDR cells for the added headway they give you (at the cost of battery longevity).
These are the go-to cells for vapers, so take a look at them, read up on the specs and performance, and make an informed decision based on what you're gonna need from them (higher Mah/higher Amp)
Sony: VTC4 (Green) / VTC5 (Green)
LG: HE4 (Yellow) / HE2 (Red) / HG2 (Brown) / HB4 (Brown) / HB2 (Green)
Samsung: 25R (Blue) / 25R5 (Green)
Hopefully that helps you a bit