In some cases, none would be the recommended maximum.

If you play with other musicicans, you'd need to keep up with whatever keys they used. If you only play with yourself ...

When my grandfather was a lad (he used to tell me), all the chaps had mouth-organs, as they were known. Most of his mates played a C harp, which is relatively low-pitched, while he played a G harp, which is higher. They could always hear him coming from afar by the pitch of his tin sandwich. Having more than one may be because of a personal preference for the pitch.

Steve
PS My mom used to play the mouth-organ, ad she can still whistle too -- pretty good for a girl, eh?