How many hairs when using shampoo in shower is considered normal? And what number isn't?

20-30 is fairly low, but the 'normal' number can vary greatly depending on your hair length and hair density. In general you don't need to worry about how many are falling unless you can see/feel a difference with your scalp.

İt is normal to shedding 100-150 hair shaft daily . That is not mean that you loss hair permanently, they regrow again . ıf you loss 300-or more in a day so it would better to go to a hair expert for checking Telogen Effluvium. 

Hair loss in the shower occurs in every person. The rougher you are with the shampoo, the more the hair loss but if you do not have balding occurring, then the total hair loss for the day in a typical Caucasian ia 100-150 hairs/day