It depends on the route - if you're flying a polar route where there is little satellite coverage, your bandwidth will be more limited. If you're flying around Europe or Oceania, speeds are better as there is more coverage.

You can tell how many people are using the system at the same time, but I haven't ever seen it go above 50 at one time - chat apps and light browsing is perfectly acceptable.

You can pay extra dollars for more quota - on the older systems there's an excess charge of 0 USD/MB