If you do a 20X21 you could divide it into 1/4s. This would allow you 3 stalls and one area for whatever you need…hay/feed storage, grooming stall…
Depending on the spacing of the supports for the carport, you could use gates as the stall front. The sides could be 1/2 walls of wood, with metal or wood studs to provide extra security for the 3 sides of the stall. The gates could be in filled with a mat or wood or tarp to provide a wind block for the colder months. Tarps Plus has a pretty good quality shade cloth with grommets that can be used to provide shelter from the sun, but still allow air flow during the summer.
If you don’t do proper stalls, you may still want to divide the shelter in 1/2 or 1/4s to allow horses to have their own space within the shelter. Sometimes you can have a real brat that won’t let anyone else under the shelter. That could be as easy as adding metal round pen fence panels to the carport. Adding shade cloth on the sides can ensure that all sides are shaded, so no horse gets left out. I used the shade cloth on my lean to/ run in on my barn on the 3 open sides to ensure everybody had a good place to be. The morning sun on the east side and the setting sun on the west side were awful during the summer. The shade cloth on the south side keeps it comfy on those warm days when the sun starts to get a little lower on the horizon.