Each chicken is different and each flock has its own dynamics. I’ve seen three hens crowded into one nest at the same time with a fourth hen pacing, waiting for room to get in. There were two empty nests available.
I currently have 11 hens laying with three nests available. Most eggs are usually in one nest but the other two always have a few. Most of my hens will lay their egg in the same nest every day, but occasionally I find one that is using a nest other than her regular nest.
I’ve had hens that are nest hogs. Most hens are willing to share but one would spend three hours on a nest while she was laying and would not allow any other hen to join her during that time. When a hen goes broody, some will welcome other hens in to lay with her while others will defend their nests. Each chicken is different.
I think you are making the right decision. Two should be plenty for six hens but you have the ability to add another if you see a need. You have the flexibility to handle whatever happens.