How many is too many?

I have a general rule of one seedling per module/pot, so if I sow two or three seeds in a module/pot, then I thin them out to just one. The exceptions are things like sweet peas, peas, ipomoea, spring onion, beets etc. I also decide ahead how many plants I want and sow in an appropriate size module/pot. The whole seed packet is rarely sown at once.

As I've gotten older I've become more ruthless about it and spare seedlings get composted apart from tomatoes which are given away. There always seems to be a willing recipient for a tomato plant!