Also, a bit off topic, but I wouldn't recommend going for a crazy number of gears, because it seems to be less reliable.
I used to have a 21 gear semi-racing bicycle and never had trouble with the chains or gears in the 20 or so years I owned it.
21 is almost certainly a 3x7 gearing, very old technology:
https://www.sheldonbrown.com/speeds.html
Yes, that's true. I haven't had a problem with 3x7 or 2x7, but have had a very poor experience with 2x10. The chain often comes off the front, despite both myself and an experienced bike mechanic adjusting it. The chain just seems to be too thin to be reliable and I've even had a chain snap before. If I remember rightly, it's a SRAM Apex groupset, which is terrible reliability-wise.
I don't need that many gears where I live, with only hill on my daily commute. My current set-up is a 53T/39T chain ring, with a 11T to 25T cassette, which gives me an adequate range. It's also not real 20 speed, because one should avoid crossing the chain, so the three lowest gears on the rear, shouldn't be used with the higher gear on the front and the three highest gears on the back, shouldn't be used with the lower gear on the front, giving a total of 14 usable gears. I'm considering a single chain ring with a wider range cassette: something like 50T or 52T on the front, with 11T to 32T or 36T on the back.
My newer bike has cheaper Shimano Claris groupset 2x8 and so far seems to be much more reliable, although it has a much lower gears: 50T/39T on the front and 11T to 32T on the back, so I hardly ever use the lower gears. I'll probably change the cassette to a higher geared one, when it needs replacing.
....So for the same effort you'll go something like 10-12% slower on a "sit up and beg" versus a drop handled racing bike.
To put the power figures in perspective, a resting 70kg man puts out 80-100W of heat, just sitting still. People are about 25% efficient, so to pedal 50W you need to produce 100W resting waste heat, an additional 150W of activity related waste heat for a total dissipation of 250W, and finally the productive 50W of of mechanical energy that you put into the bike. Measured in "metabolic equivalents", also known as METs, that's about 3 METs of activity, or about the same energy expenditure as a brisk walking pace.
Surprising just how little extra benefit there is from aerodynamic positioning. Watts of delivered power is usually easier to work with, self selecting "athletes" push out about 200W continuous over 1 hour plus:
https://www.cyclinganalytics.com/blog/2016/10/comparative-statistics-for-females
Most motivated people get up around the 100W level quickly (within weeks of repeating the activity daily).
I consider 10% to 12% to be a considerable amount. I've haven't done any calculations, which probably underestimate energy consumed slightly, unless one in the gym or on a track, due to stopping and starting. I also don't time myself too accurately because I fear it will encourage me to do stupid things to get a faster time.
I'm not a fan of calorie counting because basil metabolic rate varies considerably, even given the same BMI, sex and age and it's just impractical and unhealthy. I see it as a rough estimate: something which is good to be aware of, but not to be relied upon.