For the Guild that my LAQ attends, she coordinated a 4) swap one year. Each month it was a TOT black, with a different colour mixed in. Not sure how many in each packet for the swap. So at the end, each had a mountain of 4Ps. With the TOT Black, for many this was a creative challenge, as it was not their normal working colours.
4Ps were chosen so that if they were not done to precision, they could all be trimmed easy enough to whatever size the recipient chose. Good idea! as we all know that when a 12-1/2" block is done, that some arrive with total precision and some ... well, let's just say, not so precise.
From there ... a lot of different projects could be built. Some planned them during the year. Some waited til the end to find the inspiration.