As an Australian, I’d also argue that an infestation depends on the species of spider, and how far out of the city you are.

10 daddy long legs (cellar spiders) is a bad time if you have to walk through them, but it’s not an infestation, I think I’d need 20 daddy long legs and a few hundred little babies before I say something is infested.

1 red back and an unhatched egg web would count as an infestation for me because I’m currently living in the city, but growing up in a regional town, you’d need 5+ before it’s infested.

I’ve got about 8 chubby bum garden spiders living under the capping of the colorbond, but the fence isn’t infested because they’re just garden spiders and they’re in the garden. That’s just where they live. I feel like I’d need 50+ spiders on the fence before I consider breaking out the mortein.