roztwór nasycony

When the two mix together it is called a saturated solution.

That means you made a saturated solution (depending on circumstances, even a super-saturated one).

You now have a saturated solution at room temperature.

When it dissolves in an almost saturated solution, however, the process is endothermic.

Thus, a saturated solution creates a buffer with standard pH.

For example how much ammonium sulfate it would take to form a saturated solution in water?

The process of forming a saturated solution in this case is endothermic.

Would soaking them in a saturated solution be sufficient?

At 15.5 Celsius, a saturated solution of brine can hold 26.4 % salt by weight.

If you do not use a saturated solution, then your crystals will not grow quickly.