Thankfully, someone else has already worked out the optimum number of plants/seeds for (almost) any vegetable and soft fruit. Information is readily available on the internet, but I also use the book Grow all you can eat in 3 square feet as my go-to source. I bought a cheaper copy from Amazon.
It doesn’t matter if your bed is raised or not, is long and thin or is short and wide, just as long as you can reach all the squares without standing on the soil or over-reaching.
Each square foot (in reality one-foot-square) will sustain a certain number of plants:
One only of – potato, pepper, aubergine, celeriac, celery, Florence fennel, cauliflower, broccoli, Brussels Sprouts, kale
Four – chard, kohlrabi – I’d add runner or climbing beans in here trained up a 4-cane wigwam, one bean per cane.
Eight – peas – plant more if you just want pea-shoots, and use pea-sticks for support (fine bushy twigs)
Nine – turnips, garlic, beetroot,
Twelve – onions
Sixteen – radish, carrots, parsnip,
So a bed of sixteen squares could be all potatoes and related plant types – first and second earlies, plus maincrop, 1 row of 4 squares for each( that’s 12), plus peppers and aubergines in the final row. Tomatoes are also related – one per square.
Crops that don’t fit into the usual crop-rotation plan, such as sweet-corn (which must be planted in a grid), and squashes (which like to spread themselves about), can be planted together – one each per square. Add a climbing or runner bean into the mix and the bean will be supported by the corn, the Three Sisters method (you don’t have to have a grid garden to try this).
I’ll add to this list as I find more details. I might even make a spreadsheet!