How bad is it if one lymph node is postive after synovial sarcoma removal from left calf?

IIt is not: A common dx, unlike its name, does not always occur at joints, but is lumped as a "soft part sarcoma" so it can metastasize to lungs, but unlike many sarcomas, it can also travel to lymph nodes. Resection free margins is key. Even 1 node conveys higher risk of relapse.

Not good, but...: Unless your surgeon / oncologist tells you otherwise, your odds of long-term survival are still probably better than 50%. Since synovial sarcoma has a distinctive molecular lesion, there's a reasonable chance that within the next few years, a specific inhibitor "magic bullet" may be discovered as has happened for similar cancers. I admire your bravery and wish you luck.