Estimating the size of any given raid from contemporary evidence can be quite difficult - there are so many small-scale raids, raids in places with poor textual sources, and raids that occurred at the same time as much bigger armies that we are sometimes lucky if we get a mention that there were Norsemen in a given area outside of the main Great Viking Army [note: i don't use heathen, OE heothenas, as part of the name in order to de-emphasize a "clash of religions" narrative]. Therefore, the best way to estimate the size of a raid is by the size of the ships used - when numbers are mentioned for those, it is very small scale. The so-called A-S Chronicle mentions that Aelfred the Great won a sea battle in 875 against 7 ships, capturing one. In the same vein, in the year 897, where Aelfred commissioned a new navy "twice as long [as a longship]". The chronicle says the inaugural event was capturing 6 ships in Devon during a raid, and the later Annals of Henry of Hovenden says that in the first year, they captured or sank 30 Danish ships.

Within Scandinavia, the later sagas (and by later, I mean from the 1200s or 1300s, telling stories about the 900s) have a trope wherein a young Icelander is given a ship by Norwegian nobility to go earn renown. Njals saga is perhaps the most famous account of this - Þráinn Sigfússon is given 5 ships by Jarl Hakon Sigurdsson to hunt down an outlaw (and do some raiding on the side). Meanwhile, Grimr and Helgi Njálsson share a single merchant vessel, or Knarr, on their voyage to Iceland, which lands around the Orkneys and they immediately get into a fight on. This isn't super reliable as a source for the Viking age, but it does appear to confirm the contemporary evidence - most raids were between 1 and 5 ships, with more than 10 being very unusual.

So, now we can turn to how many people that actually is - luckily, we have a few ship burials that can help us! The Gokstad and Oseberg ship burials both represent the large end of longship burials, measuring 76 feet and and 71 feet long, respectively. The ships, before they were used as the burial site for nobility (a local chieftain for the Gokstad ship, and two women in the Oseberg ship), had room for 16 oars on each side - presumably, one man per oar. Both ships are 16 feet wide, which would allow a fair amount of space for additional people to fit in. These are, however, unusual in their width - the Skuldelev 2 ship (over 90 feet long) was just 12.5 feet wide. Skuldelev is the site in Roskilde Harbor, Denmark, where 6 Viking-Age ships were deliberately sunk - of the 3 warships, the smallest (Skuldelev 5) is only 56 feet long.

So, some estimates need to be done - warships can generally fit more people than they have oars for, so we can say 30 for the Skuldelev 5 ship, and perhaps closer to 75 max. for something like the Gokstad ship. This does assume a warship - if a merchant knarr is used tor raid, then the numbers could be even smaller! Still, averaging 50 people per ship - raids could be anywhere from 50 to 500 men, and usually sub-250. After all, any people taken as slaves, or pounds of silver given as tribute, had to still fit on the ship.

Now, I want to conclude this answer by pushing back on the idea that Norse raiders were enslaving entire towns willy-nilly. This did happen, on occasion, don't get me wrong, and the enslaving of other people individually happened all the time. However, raids usually didn't result in the total destruction of the settlement! Even the famous Lindisfarne raid in 793 didn't result in the destruction of the monastery - it wasn't abandoned until 875! This makes it immediately, obviously clear that there wasn't total depopulation, and that the monastery was not stripped bare (the Lindisfarne Gospels, which have a silver-encrusted cover, was one of the treasures brought with when the monks of Lindisfarne relocated to be farther inland).

Instead, the Old English poem *The Battle of Maldon (*commemmorating a battle in 991) suggests a very different account of how a generic raid might go. The Vikings arrive, and are met by whatever militia is available along with the local ealdorman, reeve, or town representative. There would be a negotiation phase, where if treasure is paid, the Vikings would just leave. This is accompanied by threats of extreme violence. If the defenders respond in kind, there would be exchange of arrows and stones to try and do some early damage, and then there would be a fight. There's about even odds of the raiding party losing, but if things are going badly for them, they'll retreat to the ships and go somewhere else! Additionally, if negotiations go well, maybe there's a change to trade and get valuables that way - violence wasn't the only tool in the toolbox!

I hope that helps - we are looking at often fairly small-scale fights numbering a hundred people or less. There was, however, fairly codified systems of local and elite resistance, and the wanton slaughter and enslavement suggested by their song doesn't appear to be nearly as common as one might expect.