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If you are serious about playing, I'd say two or three.

If you have just one army, there will be periods of time where the current balance state/codex/core rules just makes your army not fun to play. Having different armies which focus on different aspects of the game will help with that. Having WE, orks and BT most likely not.

For most players who cycle through more than three armies regularly, they usually lack the practice to both learn their army and play them quickly. I have quite a few friends who own 10+ armies and they always lose to forgetting rules, using specialist units in the wrong way or bad army composition, all while continuously having to look up every weapon profile.

In addition, it's an absolute chore to keep up with that many rules changes four times a year.

That's why I think one army is not enough and four armies is too much unless you play multiple times a week.