This depends on how much you try to push boundaries, as well as how tiptop your soil is.
The earlier you transplant out, the more you must plan for losses. The less time you have practiced no dig and mulched with prime compost, the more you must plan for losses.
Transplanting beetroot or lettuce in mid March in year two of no dig, you may need to bank on 50% losses needing to be replaced with back ups. Transplanting early April in year four and you may need 10% or less back ups (of course depends on how mild the spring is too).
If you want a dozen early lettuce or beetroot clumps, I would sow/prick out 18-24 into modules and keep the excess until the final transplants have established healthily.
Once you get into late April, you will likely have fewer losses (unless slugs are a problem) but may sow a good few extra to just select the most vigorous seedlings.