It’s not really the number of companies that determines this, but rather the lack of any real competition. A small enough number of companies makes this more likely, so there’s not likely a hard number of say…over 5 companies isn’t an oligopoly, they can still be - so long as they’re all focused on each other. If you see 1 company raise it’s prices and all 4 others do too, then it’s still an oligopoly. Because even though they aren’t actively getting together, and saying “hey let’s all raise our prices!”, (collusion) - the effect is the same.
It ceases to be that when barriers to entry don’t stop new competition from entering, and competition is active. (at least, that’s the simplified answer; there’s some more nuance to it, but that should at least give an overall understanding)