Acers

I have over 20 Acer seedlings from my trees, when would be the best time to pot them up into proper bonsai pots and will they be true to the parent tree/
Thanks.

Spring is the time, and yes they are the same species as the original tree

Yes it is the same species. If it is a cultivar the seadlings will not be the same variety as the parent.

Before you put them in permanent pots they should have enough character to justify a particular pot shape and color. One year old seedlings are all pretty much the same and are rarely worth relatively expensive pots.

Put them out in the sun, fertilize and water as needed, and wait a couple of years. See which ones survive- probably not all. See which ones go for height, thicker trunks, etc. A year from now maybe start shaping the trunks.

a culture in individual pot for a few years is necessary to make them grow and give them character

This all depends on what you want to do and reach.

If you have young plants, you can easily put a few bends in the lowest part of the main stem. This becomes increasingly difficult to do as the tree matures. Furthermore, repotting fairly freuqently in the first few years (ever spring) will allow you to push for a well-balanced nebari which will help long-term development.

With these two things you set the base for the tree. Then it is a matter of letting them grow and develop a trunk. Many methods exist, do a google search as you wait for the plant to grow. Development in small pots will be slow compared to growing out in the ground. It will however affect character.