I recently came into possession of a book on bicycle touring that belonged to my dad, from 1944. In it, they say (among other things) that people who have used the 3 speed bikes are reluctant to go back to single speeds, and that bikes with more gears that work by jumping the chain from cog to cog seem to work well if "meticulously set up and skillfully operated".

More gears are (almost ) always better, especially if there is no penalty in durability or ease of use.

IMO, how many and what gears you need depends on the terrain you are riding, your strength, and how much of a hurry you are in. By your (OP"s) sign-in, you have multi speed bikes, so you should have some idea what gearing range you need for the bike's intended use. After that, I would think the determinants would be the cost and the gaps between the gears.