http://www.m-w.com/dictionary/datum

data...

The plural of datum (in English) is data; and the singular of data is datum.

In German: Datum/Daten

(You didn't say which language)

http://www.m-w.com/dictionary/datum

data... although datums apparently exists as well.

But "number of data" sounds wrong, "number of datums sounds" more like it, I guess, or would noone ever say that?

"Number of data" does sound wrong; instead, you say "amount of data". And for the singular, i would say "one piece of data".

I can't say "amount of data", because I'm writing about the number of samples, eg after measuring for 10 seconds at a sample rate of 1 sec, I get 10 results. So I can't say "the amount of data is 10", but something like "the number of datums is 10".

Then I would say "10 sets of data". "10 datums" sounds really wrong to me. But what do other native English speakers think?

I wouldn't write '10 datums', but the 'number of datums is 10'.
Actually in my report I'm writing 'the number of datums n'.

I asked a mate from Australia and he said that he'd use datums.