How many you should grow depends on how many of you will be eating them! I like to germinate twice as many as I think I'll need in case the slugs or frost get some.
Last year I had two plants and got 42 cougettes which was far more than I needed as I live on my own so I gave loads away which was great. This year I had four plants as all my seeds germinated and survived and I'm too soft to ditch plants that have made it that far (can never bear to chuck my leek thinnings, always pot them up and give them away). I still have two marrows in my fridge and four courgettes going a bit bendy and I gave away about 40 and fed some the size of canoes to my friend's pigs. So basically, you don't need very many plants! How many do you have Moley?
Butternut squashes are good to grow. They store for ages and taste gorgeous. Small sugar pumpkins too.