There's no set answer to that, it depends on how fit you are, and how heavy the going is.
Usually, the best gear is the one where you can pedal most efficiently. For a longer ride this usually means whichever gear that lets you keep the cranks turning at a rate of 80-100 turns/minute.
If you bicycle has three chainrings(upby the pedals), then you want to avoid cross chaining - running the chain from the biggest chain wheel to the biggest sprocket, or from the smallets chainring to the smallest sprocket.
It puts an unnescessary bend in the chain, and the gear ratios are available at other more chain friendly combinations.
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With 3 chain-wheels on the front, and 8 sprockets on the rear wheel, using a derailleur to move the chain across the toothed gear wheels on the rear wheel, you could have (in theory) 24 gears. But, as there will be overlap of gear ratios, and to avoid overtwisting the chain-line, the number of useful gears is something like 16 to 18.
Typically a triathlon bike will have as many gears as a regular road bike of a matching price point and year of manufacture, with the current norm being 11 on the rear and 2 up front. A slightly older bike might have 10, or even 9 on the rear.
Do note that the maximum number of combinations isn't the same as the max number of usefully different gears, that's usually about 2/3 of the number of combinations due to the overlap between the big ring and the small ring up front.
Or even lower, all depending on vintage. Although it'd surprise me if anyone entering a race would use anything older than 9 at the rear.
depends on the terrain you're riding in. the flatter the fewer gears you can have. Today, you'd be looking at between 18 and 22