The average number of sick days used is 15 or the number of days offered?
In New Zealand we get a minimum of 10 sick working days per year but some companies offer more and allow unused sick leave to accumulate.
Sick days are not “offered” by employers. Sick days are prescribed by the doctors and there is no upper limit. After all, your sickness will not disappear just because it has been N days. That's at last how it is in Poland.
Even the concept that you need permission from your employer to take a sick day is crazy to me. After all, if you're sick, you're sick, not like a hard deadline of 15 days (or whatever) is going to make the sickness go away?
I wrote a primer about sick days here: https://allaboutberlin.com/guides/sick-leave
15 is the average. I use it to reassure people that it's okay to take sick days, and not one of those rights that no one dares to use.
Usually, employers ask for a doctors' note after 3 consecutive sick days, but the reason for the sickness remains hidden from the employer. The note just gives a time range, nothing more.
You don't have an offered number of sick days in Germany. If you're sick, your sick. At some points (after 6 weeks) the employer stops paying for it, and the payment switches to the health insurance and drops down to 70% of your usual gross salary (with some more specifics).