Just about the only electrolyte lost in sweat is sodium, and you can lose a lot. If you sweat a lot, you lose a lot of sodium. It varies between individuals and with training. It is advisable to replace sodium as it's lost when sweating profusely over extended periods or it can dip to dangerously, and certainly performance degraded levels, even if you get the recommend amount in your normal diet. Something like 250mg/hr is generally appropriate.

Potassium, on the other hand, is not lost in consequential amounts in sweat, so if you get enough in your general diet, you're good and don't need supplementation during exercise.