How many sons does Hrothgar have in Grendel?

This question is not relevant in the least because Hrothgar and Grendel are both characters in the epic poemBeowulf:

Hrothgar is the king of the Danes (Scandinavia, or Denmark). The story mainly takes place in the Scandinavian mead hall named "Herot."

Grendel is the devilish monster that represents all that is evil. Grendel is the monster who encountered the sleeping warriors/drunkards of Herot in the middle of the night and ripped them to pieces only to then immediately consume them one by one and drink their blood. Beowulf killed Grendel with his bare hands because Grendel enchanted all the sWordsmen's weapons, so they appeared blunt and could not penetrate his skin (after Beowulf's victory, he ripped off Grendel's arm and hanged it). And after Beowulf kills Grendel's vengeful mother in the moor, he beheads Grendel's dead body and brings it to the men of Herot.

So, to conclude this answer, you obviously have no grasp of the concept or any of this historical story's premise. In no place does it say anything about an amount of sons Hrothgar has.

It would be easier if you read it.