Hi Delphi,
I grow runner beans, 20 plants will easily keep you in beans for months with loads to spare. 40 plants will not take up too much room and you can give a lot away or swap them. I also grow peas, courgettes, onions, shallots, garlic, tomatoes (in a greenhouse, they all failed when planted outside) and for the first time, butternut squash, also in the greenhouse. Chillie peppers, spring onions, rhubarb, raspberries, blueberries, strawberries, beetroot and leeks. All on a 10 foot wide by thirty foot long patch of garden, and that includes a spot for the greenhouse. We do have a little trouble from a badger digging for food now and then, it made a right mess of the broad beans one year. Oh and I nearly forgot spuds and cucumbers. I have planted the spuds in pots this year as they are sometimes hard to find in the ground and you can control the soil easier. If you plant 4 courgette plants you will probably give many of the courgettes away unless you have a courgette recipe book. We also planted a grape vine 2 years ago, nothing from it yet but it looks good trained across the back of the house. Good luck.
Lee