If you pipette 200 µl of a stock solution into a 10 ml organ bath, you do a 1:50 dilution (0.2/10). Thus, the final concentration of acetylcholine in the bath will be 1/50 µM = 20 nM.

Just for clarification, is the stock solution of ACh always 1 µM?

Diethart is absolutely right. 200 uL in 10 mL equals a dilution of 1:50. Thus the stock solution of 1 uM ACh will drop to 20nM.