Walk up and down the grid of any big event, and you’ll probably see a range of gears within 2-3 teeth on the fastest guys.
Pegging the limiter just at the end of the straight isn’t always the goal. Even before you get to the limiter, the kart is running out of power on the top end.
The reality is, as with most things in karting, “it depends”. A longer, more flowing track might allow you to run well below the limiter, as you are able to lug the engine and keep it in the mid-range power for longer. A track with really tight hairpins and long straights might require you to stack rear gear to deal with the corners, and then you just have to sit on the limiter halfway down the straight.
Similarly, you might have a different line or chassis setup or driving style that will change how you gear the kart. If you drive very smoothly, you may be able to roll through the corner easier than some guys, and run a lower gear to drive by them on the straight. If you have a more aggressive driving style, maybe you can make up time on the entry to the corner by driving in harder, and then add rear gear to help dig yourself off the corner easier. You also need to consider changing grip levels. Once the track starts to rubber in, you might find you can keep dropping teeth as you are carrying more corner speed.
Also, you need to consider whether you’re going to be racing in traffic or out front of the pack. A driver out front might be able to run less gear for more top end because they aren’t battling or getting held up in the infield at all. If we are going to be running in traffic, we usually consider adding a tooth so that we can battle and have a little extra torque for pulling off a corner after a pass or for defending.
So I would say there’s a range of gears per track that will give you a good lap time. Then it’s up to you to figure out which in that range suits your driving style and will allow you to race effectively.
Don’t overthink it. Find a gear you think is right, then test within a range of 1-2 teeth down and 1-2 teeth up and see what the lap times look like. Then pick a gear for your racing situation.