So one person simply posting that they have 6 tomato plants will mean nothing in the absence of a lot more information.
Not entirely sure I agree (although you are right, in the way you said it).
I think numbers provides a ratio from one variety to the next. So if you are a family of 4, and me a family of 2, then if you grow 20 tomato plants and 40 parsnips; and if I grow 10 Toms and 20 Snips, then the ratio is the same
Of course I might love Parsnips and hate Tomatoes, so my ratio could be different.
Parsnips is not too difficult - you can work out (roughly) how many roots you might buy / eat in a season. But Runner Beans is more difficult because one-plant is not equal to one-pod ! and only experience tells you how many plants gives you the harvest you want. Some things, like Courgettes, are way more productive [per plant] than, say, Peppers or Aubergines ...
I find it useful to keep a notebook. Crop / variety name with dates sown, germinated, planted out, the number of plants and the First/Last harvest date. Plus a note of whether it was too much / too little, and whether too early / too late, and then you can modify next year. Whether you liked the flavour of that variety, or not, will be handy too.
When you've been doing it for 5 years you'll probably remember what you need / when / how much etc. and may not need the notebook
I grow:
Beans, Broad - 25 plants
Beans, climbing French - 80 plants
Beans, Runner - 25 plants
Beet, Leaf - 20 plants
Beetroot - about 10 pots with 3 seedlings each once a month through the spring/summer
Ditto Kohl Rabi
Brussels Sprouts - about 12 plants
Carrots - I grow them in pots. One pot each month.
Cauliflower - 4 every fortnight from Feb - June
Celeriac - 20 - 30 plants
Celery - 20 plants
Chinese Cabbage about 20
Courgette - 6 plants (probably too many for most people!)
Leeks - about 100 plants
Lettuce - about 6 every fortnight Spring to Summer
Onions, from seed, about 100. Garlic cloves about 50.
Parsnips - 50 plants
Mooli Radish about 20
Potato - first early in bags, 6 x bags, 3 tubers each. Outdoor first early about 30 tubers. Outdoor second early about 40 tubers. Outdoor Pink Fir Apple about 30 tubers. I don't bother with main crop.
Spring Purple Sprouting Brocolli - about 12 plants
Butternut Squash - 6 - 10 plants
Sweetcorn - 2 batches in greenhouse, 15 plants each. Then 2 outdoor batches about 20 - 30 per batch.
Greenhouse:
Tomato - 15 plants
Melon - 20 plants
Aubergine - 15 plants
Sweet pepper - 12 plants
Chilli pepper - 6 - 12 plants
Cucumber 3 - 4 plants
probably some things I forgot!