14l Black bucket is not quite as big as the pots I use, it's as if the square corners have been cut off to make a circle. In mine I grow runner beans, broad beans, climbing french beans (4 per pot), sugarsnap peas (possibly 8 per pot) cabbagey things (1 per pot), radishes (1 inch spacing), lettuces, beet, swiss chard (about 6 inches spacing), strawberries.
Tomatoes are OK in a slightly smaller pot, I use pots about 9 in dia which is what? 9l?
Cucumbers would be OK, 1 per pot.
It's too small for courgettes and squashes.
In theory it'd be OK for a couple of raspberry canes or a fig.
The advantage over a raised bed is that you can move pots around, so you can start things off indoors and move them into place once they got past the slug-prone stage. Really useful for me as I can start everything in the greenhouse. If you haven't a lot of space, herbs are a good thing to grow (fresh herbs are expensive to buy). Most won't need as deep a pot, or you could keep a bay tree in one pot (they grow easily from cuttings if you know anyone with a decent sized bay tree).
Concentrate on growing things that you can't get in the shops or which are really expensive - potatoes are fun but they're hardly going to reduce your veg bill, strawberries or sugar snap peas on the other hand give you a treat that you might not otherwise buy. And remember you can re-use the pots year after year, it's an expense in this year only.