Quite a few depending on the size of the pot? I find some of the Heucheras look good in terracotta all year round. Pelargoniums will look good for a quick summer look.

I love geraniums/pelargoniums as well, the rich colours look enhanced against the terracotta. Or, maybe plant a nice red Acer for a japanese look.

It depends entirely on the size of the pot.

For a lot of small shrubs or climbers you'll need a pot that's deep enough for the plants' rootball so about 2' deep, usually 18"-2' wide too.

If you're only thinking of annuals here then most small pots will do.

Do remember, terracotta leaches moisture so copious watering's required.

Also, if the top of the pot is narrower than the lower parts, when a shrub or vine needs transplanting, it may be necessary to smash the pot to get the plant out.

Good point.