It's possible to get four on the front, twelve on the back, and four in a hub. That gives you 192 gears. But you could times that by four, using a tandem, by using a leading four. The stoker's pedal arms would have to be removed, as the rear crank would turn out of sync. Still, that'd be 768 speeds. More if you can get more in the hub, but i don't know of one that'll go in a cassette carrying hub, perhaps the old Sturmey 5-speed goes in the generic shell. Maybe a cassette could be adapted onto an Alfine 11-speed. You'd be looking at the possibility of circa 2,000 gears.
My tandem has the 'armless crank' on the rear, but the leading crank is a single so no added gearing. In fact it's just 2x10 speed, as making a high number of gears wasn't on my agenda. However, i have a final drive on it, making those 2x10 gears longer than the tooth count suggests. This as the front ring is 38t and it pulls the 30t granny ring on the rear triple.
I also have a final drive on my recumbent trike, enabling me to have a small chainring up front. That has both hub and cassette, but no additional gears up front. And an outboard 250w motor pulling the rear chain, so the motor uses all the gears (13-34t cassette plus 144% 3-speed hub)
The most gears i ever had was 81, using a 9-speed cassette on a 3-speed hub, with a triple up front.