Are there any risks?

The answer to this is short and sweet: NO.
Neither your health, nor your fertility are negatively effected when you donate sperm. One could even say: quite the contrary. After all, you are subjected to regular check-ups which means any problems are detected rather sooner than later.
And emotionally you have nothing to fear either. Sperm donation is anonymous: the recipients do not know who the donor is, and vice versa.
And finally: only a restricted number of children are fathered per donor which means there is no risk of consanguinity with your own children.

And... do you get anything in return?

Yes and no. No, you don't get any money for your sperm. But in accordance with Belgian law, you will receive an expense allowance.

In theory there is no limit to the number of times you can donate semen. However considering the fact that only a restricted number of children are fathered per donor, this does imply a restriction of the number of sperm samples that can be donated.