You cannot compare mg to mg of different medications, as they're all different. Some medications start out at 0.5mg, some at 100 mg, some at 500 mg.
50 mg of Zoloft is on the low side of normal. 200 mg is the max recommended daily dose. I'm on 150mg/day, for panic disorder.
Try not to get hung up on numbers. What you have to look at is how you're feeling and if you think it's effective. How long have you been on it again? Remember too that it takes a while (4-6 weeks) for you to adjust to it before you would notice significant improvements.
You have room to go up if needed. You basically just have to wait and see. You will get to the point where you will find an optimal dose. If you can get away with the Zoloft being effective at a lower dose, that's great. I'm a believer in less is better, starting low and working your way up.
Also, just an FYI, it's not at all abnormal to need an occasional dosage increase from time to time, as the medications start to lose their effectiveness to a degree. My experience was every couple years or so. I've been back on Zoloft for a little over a year, it took me a few months to adjust and find the best dose for me (150 mg), and I've been good since. I haven't needed to change a thing.
Hope this helps.