If your line is 20 Mbps, then anything above that can saturate your capacity. Which means that a faster firewall is no better than the lowest speed firewall that can handle your traffic.
So if you are looking at Gigabit firewalls, I would presume that you either need their features (two lines) or that you are intending to speed up your connection at some point in the future.
But no matter, it sounds like even if you got Gigabit service a 1.5Gbps router would handle it easily.
Note that when they tell you those kinds of speeds, those are "marketing" speeds. In actual use, your mileage may vary.
Second answer: The best way to estimate the total user need is to look at the types of things they do.
For instance, if they use web pages to fill out forms, that is "bursty" and the long-term bandwidth usage is low. First the page loads, with graphics, etc., then there is little activity while either filling out forms or even reading the news.
Streaming users require moderate bandwidths for extended periods. I use a rule of thumb of 1.5 Mbps per full-screen streaming user. HD is higher, of course.
Then add it all up.
I always then double it.
Then compare that to your internet capacity. (since your firewall is so fast it is like it isn't even there).
By doing that you can decide whether you may need more speed from your ISP.
The trend is towards more video (think Facebook Live), so that should be part of your estimates of how much speed you need.