There are details missing in this question that matter tremendously. Squirrels are faster and more agile than us. If they are well coordinated, and behave optimally to win (without concern to their individual survival, only the group’s success), I think it would take only a small number of squirrels to brutally murder most people, something like 5. I think their best strategy would be to go for the eyes first, then inflict bleeding injuries and escape again before the person can react. Without tools, and without backup, this approach wouldn’t take long to wear down most people.

If the squirrels don’t care about their own survival, but make straightforward attacks, I’d think closer to 10-20. The person’s injuries will still compound quickly, but once thet have a grip of a squirrel, it wouldn’t be especially hard to lethally injure.

If the squirrels still behave like squirrels, and are instead attacking because (for example), they are starving, then the number probably doesn’t matter much, as they’re more likely to go after each other, and the person would have the opportunity to plan and ambush small groups at a time.