Find out how many users it can take

Raise the number of users in stages and watch where latency or errors start to climb. -stages takes users:time pairs: the first number is how many users to reach, the second is how long the stage lasts. The users go up smoothly during each stage.

vegaload run -target http://127.0.0.1:8080/widgets -protocol http1 \
  -executor ramp -stages 10:20s,25:20s,50:20s,0:10s

This run lasts as long as its stages, 70 seconds here. -duration is not used with -stages, even though the start line of the run prints the default 10 seconds. Open the HTML report and look at the time series: the point where p95 bends up is your limit. Use -executor step for jumps instead of smooth climbs.