We don't know for sure. The main source on it we have is the anglo saxon chronicle which says

on the sixth day before the ides of January in the same year, the harrowing inroads of heathen men made lamentable havoc in the church of God in Holy-island, by rapine and slaughter.

So we know that typical early viking age raiding was done by a small number of ships in a given raid which slowly increased over about the next eighty years with the size of raids becoming larger along with their frequency until they began to be able to bring a large enough force to achieve more permanent invasions with larger ships and more of them. But early on these men were looking for soft targets that offered good plunder at a coastal location that they could hit without too much of a fight and retreat before a defence and counter attack could be organised. The exact number of ships we don't know. A typical raid did involve multiple vessels but usually you're looking at about half a dozen or less. If nothing else an excessive number of raiders reduces the possible rewards per person, so typically speaking you'd take only as many raiders with you as you'd require to deal with any resistance from the kind of target you were aiming to find.

We do know the next year's raids definitely involved multiple vessels as the same chronicle says

In the meantime, the heathen armies spread devastation among the Northumbrians, and plundered the monastery of King Everth at the mouth of the Wear. There, however, some of their leaders were slain; and some of their ships also were shattered to pieces by the violence of the weather; many of the crew were drowned; and some, who escaped alive to the shore, were soon dispatched at the mouth of the river.

Using the plural