If you have a legal business entity, you can do as many different kinds of business under that name as you want, but they all share the risk. Meaning, if someone falls and breaks their leg on your rental property, if one business owns all your rentals and your web business besides, it is all at risk in a lawsuit. Only when you need to isolate risk is it worth it to have multiple business entities in my opinion.
The usual advice has been in the past to start as a sole proprietorship and then incorporate later. This advice is changing, as it is really super easy (and cheap) to start an LLC these days. I started as an LLC. Usually it costs about a hundred bucks and doing some paperwork once a year, and that's it. And from the IRS' point of view, an LLC can be treated as almost anything else, so a single-member LLC can just be a passthrough entity where everything ends up on your own tax return just as a sole proprietorship would.