How can I know how many days my holiday

I work 10 months how many days my vacation that I get?

If you work 5 days a week, you will earn 1.8 days per complete working month. This adds up to 20 days of holiday a year. Similarly, if you work 6 days a week, you earn 2 days of leave per complete working month, adding up to a total of 24 vacation days a year.

Normally you have work for 12 months to accumulate the holidays then after 12 months you can start using it. This is for 38 hours in one week. If this is your first year you are allow to request "European holidays" only applicable first year.. The holidays you will take will be covered up by your vacation money (not recommended).
Technically you don't have to accumulate 12 months.
It's more how much day did you work per each month until the 31th December, that give you the number of the holiday you get for the next year.

Student finish their studies in June, start working in September. September to December is 4 months.
==> 4 * +/-2 = 8 paid vacation days next year.