Mesembryanthemums tend to spread between 9 and 12 inches. You may get away with two or three plants squeezed in together. If that seems too much when they mature then take out what isn't needed, trying not to disturb the roots of the others.
They are perennials but I doubt they would survive a winter outside due to frost freezing them in the pot. Perhaps you could over winter them in a shed or garage but don't let them dry out completely, give them a small amount of water when necessary.