Hi Chalky and welcome to GoY, which particular seedlings are you asking about?

None. As long as the seeds have been sown in decent soil or compost, the seedlings won't need feeding.

I thought seedlings should be grown in unfertilised soil, or is it just seeds?

If sown in a sterile seed starting mix, they might need a half-strength feed with a liquid plant food after they form one or two true leaves. Either chemical or organic types work, though organic ones, such as fish emulsion, are usually gentler.

If sown in a sterile seed starting mix, they might need a half-strength liquid feed once they grow a true leaf or two. Either chemical or organic ones will work, though organic types are usually slower and gentler.

I grow seeds in a mix of compost and lots of grit. That makes it easier to pull the seedlings apart when they have produced two true leaves. I understand that is when the need to be potted on in to individual cells or pots. I use potting on compost which should have enough feed to last for at least six weeks. As Mg says it depends on the seeds you are sowing. Packets usually tell you how to sow the contents. A friend told me if I was sowing lots of donated seeds a good rule of thumb is to fill the pot you want to use then depending on the size of the seeds, sprinkle them on top and gently push them more securely on to the soil, don't cover or cover very lightly. For larger seeds make a hole with a finger and pop all the seeds in together. For seeds around about a quarter of an inch in diameter the depth needs to be about 1". Seeds germinate at different rates so it could take more than one growing season. Do not feed them, they will come when they are ready if the seed is viable.

Sorry about doubling up: my computer didn't show the first comment after I posted it.
Here in the States, we frequently start seeds in a sterile mix of peat, perlite, and vermiculite. It rarely contains damping off fungi, but it also has virtually no nutrient content, so seedlings sometimes need a feed or two before they are big enough to transplant to a richer mix.