The only limit you would have is the resistance (ohms) of your coil as the coil resistance is your only other safety and throttle along with what battery you use, you already listed one variable that matters, the battery (Samsung 25R = 20amp Continuous Safe Discharge Rate at 2500mah). So a 20amp load is the maximum the mod, RDA, and battery all together can handle. 4.2v is the fresh charge on a battery, and knowing a mech doesn't regulate the power output down without any restriction, what the battery has available fresh charge is the critical state, we use this formula -> Voltage/Resitance=Final Amp Draw (and as the battery slowly decreases from 4.2v down to a minimum safe voltage of 3.0v suggested lowest discharge, here are some examples to show you.
4.2v/1.0ohms=4.2amps
4.2v/0.5ohms=8.4amps
4.2v/0.25ohm=16.8amps
With Mech+RDA combinations when I run them, the lowest I ever build is 0.25ohms, though my general builds are in the 0.4, 0.45, to 0.5ohms range (about 50% the maximum CDR the battery can sustain), reasoning behind this, as a battery ages due to standard use, charging, high stress (pushing toward its maximum discharge rating), and the biggest killer is heat which high stressing causes, the Mah and C rating of the battery decrease, these 2 stats determine a battery's CDR, so six months of above average stress, charge and discharge, a 20amp CDR battery could in technicality be only a 10amp CDR battery when the dance is done.
1) Get a good Ohm Reader or Digital Multimeter to check your Ohm reading before ever firing a coil on your mech, get a good grasp of using the "Ohm's Law" formula above to test if you are being safe or not.
2) Bookmark this ->
www.steam-engine.org <- Has a lot good tools for vaping including an Ohm's Law Calculator and a Coil Building Simulation Calculator
3) Be safe out there
Resources also check out
@Mooch <- Resident Battery Model Tester, his charts are a great reference to stay safe
@Baditude <- His Blogs have a lot of great, valuable information for everyone from beginner to advanced.