Two apple ID's

Personally I would use 1 Apple ID and manage your businesses through separate email accounts. It would also depend on how many devices you use for your businesses and for you personally. If you had a separate ID for each business, then you could potentially be looking at 3 Apple ID's if you used one for a personal phone, which I think is unreasonable. If you use just one iPhone for your businesses and personally, than most definitely you would want just one Apple ID.

With that said, I do have 2 Apple ID's. One is for my developer business which I use a separate iPhone for testing and a separate Mac for developing. The iPhone does not use any services, such as Apple Card, Apple Music, or Apple TV. On the personal side, I have a Mac and an iPhone that does use all of the Apple Services.

The benefit of using the single ID in your similar businesses is that you can share the resources such as passwords for websites, notes, documents stored in iCloud, and contacts a lot easier. For the Contacts, you can create a separate folder for each business and do the same for Notes, and documents stored in iCloud.

It is really up to you and what you find most convenient. One way to look at is is Apple's design which is a single Apple ID per person. If you want to keep things completely isolated from the other business then go with multiple IDs. If there's overlap then maybe just one ID. It isn't easy to share purchases between IDs unless you set up Family Sharing. While you could use that with businesses, the purchases would be shared which might make accounting messy.

As you weigh your options, keep in mind that iPhones and iPads are designed as “Single AppleID” devices.

While you can change their assigned AppleIDs, it’s simply NOT practical to do so on any sort of routine basis. (and certainly not-at-all “on the fly.”)

Macs have a bit more flexibility, but even they would normally have to use separate local user accounts.