Misspendragon,
If your concern is physical stamina, you can work in that by either consistently having double lessons with the intention of sustained work. Or, you can actually go to the gym, work out, take Zumba (and put your heart in it) etc.
If it's mental stamina -- which is a real thing at comps, then, yes, you might need to do this at comps. Or you might not need it at all. Some people have it from past experiences doing other things. If you need it, doing lot of singles might do fine for you anyway. It will mimic the "up-down-up-down" cycle for adrenaline. Also, even if you mosty focus on one dance type (e.g. Smooth), if you know two, do two at the event. You'll get more "up-down-up-down" that way. It will cost more though. So you need to balance that (assuming you have a finite budget. I shouldn't assume because some people seem to have infinite budgets!)
I don't find physical stamina an issue. I did have mental jitters at my first competition after all the Covid seclusion. It was just weird being around lots of people! I don't think I really even needed a comp to get over that-- just being around people more which is happening gradually anyway.