To start things off, there really isn't a such thing as a strict "limit" per se: there's ranges that are considered very safe, safe, and increasingly dangerous, respectively. This depends primarily on your initial limb lengths, what type of lengthening you're doing, etc. Lengthening 10-15% of your tibias and femurs is considered very safe, with 15-20% considered relatively safe, and 20%+ considered more risky. However, you could still lengthen 10 cm per segment and be fine: people have done it before, though it is risky territory and will require a long recovery. Should you be one of those people? I wouldn't do it, and you shouldn't unless you're in fantastic health, young, and have a top doc--even then, I'd be worried.
Now, back to your situation: measure your tibia and femur lengths according to these videos, then see what you get.
After doing that, you should peg your maximum at 15-20% of those limb lengths. I heavily advise starting with the tibias, seeing how you recover, then doing femurs if you still feel dysphoria about your height. Externals are best, but lengthening over nail or lengthening and then nailing are both great, too (I'm considering the latter).