In both my preschool and kindergarten classrooms, I had to find a way to keep my centers organized. During free exploration time, I didn't want a ton of students in one center. So, to limit the number of children in each center, I did two different things.
In my preschool classroom there was a large bulletin board. There was a picture of each center, with a label, and pieces of velcro under it. The number of velcro pieces indicated the number of children allowed in that center. Each child had a small piece of tag board and their picture on it. On the back was a piece of velcro. When they wanted to go to a center, they would stick their picture card on the other velcro piece under that center. If there wasn't a velcro piece for them, that meant the center was full and they had to make another choice.
When I taught kindergarten I had a smaller classroom. Instead of a choice board, I had the children make beaded necklaces, each a different color. The necklaces were hung on a 3M hook in each center. The number of necklaces hanging indicated how many children were allowed in that center. If there wasn't a necklace, there was enough room. I kept the colors in each center the same. So pink was for dramatic play, green for science, white for the water table, etc.
Supplies for choice board
Supplies for beads
Lots of beads
String for necklaces