In 2023 I paid mor than 15K in daycare - but my deduction is only for $600 - should be more? 20% with a maximum of $3K?

It sounds like you might have Dependent Care Benefits in box 10 of your W2.

How many children do you have?

You can use up to $3000 of expenses (for one child).  That means you are using $3000 of expenses, multiplied by 20%, which is $600.  Everything sounds correct.