Is there any way to know the body count for that battle for one man, however famous he is?

We don't know for sure. The Anglo-Saxon Chronicle is terse: "But there was one of the Norwegians who withstood the English folk, so that they could not pass over the bridge, nor complete the victory. An Englishman aimed at him with a javelin, but it availed nothing. Then came another under the bridge, who pierced him terribly inwards under the coat of mail." No body count is recorded.

Our main source for the Norwegian side of the story, King Harald's Saga, describes the battle in more detail, but mentions no such bridge incident. (It does describe the death of Harald due to an arrow in the throat, while fighting on land, rather than alone on a bridge, so the bridge-defender was not Harald.)

I don’t know much about Viking age weaponry, aside from what I’ve seen in various media, but how realistic is it for one man to cut down 40 men as he covered the retreat for his forces?

In principle, it is possible, as long as the attackers were careless of their lives, and continued to queue up to fight one after another, despite being killed one after another.

I know that in history, there are other cases of exceptionally determined warriors blocking a bridge to let those on the other side escape, but how often would that work? How long could someone hold out against overwhelming numbers? In practice, it is likely that the death of the first few attackers would discourage others, and javelins and archery would be tried. This is likely to result in a much smaller body count, but will still delay the attackers.

The Anglo-Saxon chronicle reports it as an exceptional event. Perhaps the other two most famous cases of defending bridges are Publius Horatius Cocles, of "Horatius at the bridge" fame, and the Minamoto defence of Uji Bridge. For the latter, from the Tale of Heike,

Then Gochiin-no-Tajima, throwing away the sheath of his long halberd, strode forth alone on to the bridge, whereupon the Heike straightway shot at him fast and furious. Tajima, not at all perturbed, ducking to avoid the higher ones and leaping up over those that flew low, cut through those that flew straight with his whirring halberd, so that even the enemy looked on in admiration. Thus it was that he was dubbed "Tajima the arrow cutter." Another of the soldier priests, Tsutsui-no-Joinyo Meishu, wearing armour laced with black leather over a hitatare of dyed cloth, and a helmet of five plates, a sword in a black lacquered sheath at his side and a quiver of twenty-four black feathered arrows on his back, his bow being also of black lacquer, gripping his favourite white handled halberd in his hand, also sprang forward alone on to the bridge and shouted in a mighty voice: "Let those at a distance listen, those that are near can see; I am Tsutsui Jomyo Meishu, the priest; who is there in Miidera who does not know me, a warrior worth a thousand men? Come on anyone who thinks himself someone, and we will see!" And loosing off his twenty-four arrows like lightning flashes he slew twelve of the Heike soldiers and wounded eleven more. One arrow yet remained in his quiver, but, flinging away his bow, he stripped off his quiver and threw that after it, cast off his foot gear, and springing barefoot onto the beams of the bridge, he strode across. All were afraid to cross over, but he walked the broken bridge as one who walks along the street Ichijo or Nijo of the Capital. With his naginata he mows down five of the enemy, but with the sixth the halberd snaps asunder in the midst and flinging it away he draws his tachi, wielding it in the zig-zag style, the interlacing, cross, reversed dragonfly, waterwheel, and eight-sides-at-once styles of fencing, and cutting down eight men; but as he brought down the ninth with an exceeding mighty blow on the helmet, the blade snapped at the hilt and fell splash into the water beneath. Then seizing his dirk which was the only weapon he had left, he plied it as one in the death fury. Now a retainer of Joen-bo-no-Ajari Kyoshu, Ichirai Hoshi by name, a man of great strength and courage, was fighting behind Jomyo, but as the beams were so narrow he could not come alongside him, so placing a hand on the neckpiece of his helmet, he shouted: "Pardon me Jomyo, this is no good," and springing over his shoulder to the front fought mightily until he fell.

Here, three warrior-monks fought to stop the bridge from being crossed. Ichirai was killed, and no body count is given. Jomyo is described as killing 15 enemy on the bridge (and another dozen beforehand, with his bow). This was considered an exceptional feat! Jomyo escaped with arrow wounds, saved by Ichirai's intervention.

As for Horatius, the Roman accounts differ somewhat, including whether he survived or not. One thing that multiple accounts report is that his defence of the bridge was an exceptional feat. As Livy put it:

he leaped down into the river, and under a shower of missiles swam across unhurt to his fellows, having given a proof of valour which was destined to obtain more fame than credence with posterity.

i.e., it was a pretty incredible feat.

These incidents are preserved because they were so exceptional. If they were more ordinary, more likely to happen, many more such cases would have been recorded, and they would not be such rare heroic feats. These events might seem incredible to us today, and they also appear to have been fairly incredible to people in the past.