In order to achieve the yields you desire from your garden, you need to be sure you have enough seedlings planted to generate the number of mature plants you desire. Losing a month is often not an option, so a bit of maths is worthwhile.
The maths goes something like this:
Number of strong seedlings created for transplanting = number of mature plants desired + number lost after transplantation (slugs and other pests, weak young plants etc)+margin of safety due to unforseen events.
Number of seeds to sow = (number of strong seedlings for transplantation/frequency of seed germination)+excess required to eliminate weak seedlings.
Obviously the better gardener you are, the fewer seedlings you will lose following transplantation and the fewer weak seedlings you will create after germination.
However, everyone can be honest with themselves currently, whilst striving to improve in future.
When I started growing tomatoes, I did not get good germination and I lost a few potting up. After four years of saving my own seed, I get 100% germination and almost never lose a plant through potting up. I still retain one spare in 8cm pots, allowing me to pick the best two of each strain.
When using commercial seeds, there is more uncertainty. I have sown two batches of Alderman peas in modules in 2017, firstly in February for shoots, then in early April for maincrop peas. Both sowings have shown a 65-70% frequency of germination and I used that figure when estimating how many seeds to sow to generate 90 seedlings for my 3 metre row. I sowed 160 and 12 days after sowing, 103 have germinated.
The other time honoured solution is to sow two or three seeds per module and thin to the best one as desired.
When creating clumps of seedlings (e.g. for onions, shallots and salad onions), I usually put 6 or 7 onion seeds looking for 5 mature plants in the clump; 5 or 6 banana shallots looking for 4 plants; and 10-12 salad onions hoping for seven or eight mature plants.
The other major issue is season’s beginning or end. When pushing the boundaries, more plants are likely to fail, so more back ups are a prudent precaution. I progressed 10 spinach clumps for eight weeks to ensure I had five successful clumps in the ground by mid April. Similarly, I progressed 15 beetroot clumps from a mid-February sowing to achieve 10 healthily transplanted clumps in mid April.
Every vegetable is slightly different in this regard.
But spending a few years working out effective margins of safety to ensure you have good crops almost every year (there is no plan to counter a freak snowfall in May in Greater London, after all, other than hoping as many plants as possible survive under fleece) is definitely worthwhile.