The size of the Norse raiding band as well as the mobilized army itself in Early Medieval Europe has been hotly debated topic among the historians especially since 1960s (Halsall 2003: 119-33), and they have not probably reached an agreement on the topic.
Generally speaking, the number of the crews in a Viking ship could be measured by the oars or the 2 row bench for oars in written text and in archaeological find (ships themselves): While most ships mentioned in medieval Scandinavian law books had either 15 oar benches (i.e. 30 oars) or 20 oar benches (i.e. 40 oars), some large Norse warships from the 11th century had 60 and more oars (Bill 2008: 178). The crews corresponding with 100% of such oars would be ideal to handle this kind of ship, and the ships could sometimes have warriors on board more than 150% (even maximum ca. 200%) just for a naval battle. In short, each Viking ship could muster ca. 30 to max. 120+ crews, though as for raiding (not the pure naval battle) purpose, the numeral figure of crews was likely to be closer to those of oars (ca. 30 to 60) or even less, I suppose.
So, the next question concerns: How many ships the raiding band of the Vikings usually consisted of? The primary texts on the victim side either in the British Isles or in Continental Europe often record hundreds of ships, and scholar have not agreed on how much the authors of these sources exaggerated the numbers. As a modest outline, Halsall suggests at most a few ships (i.e. around 100 men) for the early 9th century (sporadic raiding against the settlements as well as ecclesiastical institutions) and ca. 6,000 men for the large-scale fleet (of ca. 200 ships) for the late 9th century like the 'Great Army' (Halsall 2003: 123).
From a point of view of Scandinavian history, however, I would like to make a notice that to mobilize a fleet of ca. 200 ships was rare (notable) and difficult even for the rulers of high medieval Scandinavian kingdoms that had probably more than double of their population size of the Viking Ages, as I briefly summarized in this post before.
References:
I'd recommend reading Sawyer for this (as, indeed, for any general introductory question on the Vikings).
He notes an important distinction between where the attacks were coming from/going to. Longer sea crossings made keeping together attacking fleets harder, necessitating smaller groups. He argues, then, that attacks from the coast of Denmark onto nearer targets could practically be substantially larger - against the coast of what is now France, for example.
It's also fairly safe to assume that the scale of attacks varied dependent on the political context. Viking raids into Frankish territory captialised on periods of internal instability (eg. the civil war) that lead to an inability to fend them off/maintain defences. Where early raids against places like Lindisfarne had been relatively small, the recognition that the Vikings could get upstream to relatively unguarded ecclesiastical sites, and that people were willing to pay them to go away, encouraged more to come.
In terms of actual numbers, Sawyer throws around a few figures. A fleet of 13 ships is said to have been combated at Flanders (and I'd refer you to u/y_sengaku 's answer below, who knows far more than I do, for reference to that in real people terms). It's worth noting, however (and again this is Sawyer rather than my small little brain), that the reference to 200 ships attacking Frisia in 810 mentioned by the Frankish Annalists is most likely a hefty exaggeration.
Reference: Sawyer, P. H., Kings and Vikings (New York 1994) - entire book, but particularly pp.80-81.