R12 wrote:I have a single cabbage plant in a pot. ... Do I have to put a rubber band around it or will it come naturally?
IF it makes it will come naturally.
Tying leaves around a (center) head is called blanching. Blanching is commonly done for cauliflower but not cabbage.
IF it makes a head please post the picture before you harvest it.
I would be pleasantly surprised if it makes in a pot, especially one that is smaller than the plant. If like other Brassica it has a pretty healthy taproot which would be difficult to have|grow except in a very large|deep pot.
[ For reference ]
I dug & pulled (organic) collard plants this year after they had gone to seed. Most of the quite large(2"+ diameter), tapering roots were well over a foot long ...