Looking at the features of several hosting companies, they say things like unlimited mysql databases. I’m wondering why I would ever need more than one (for a single application). Is there some kind of size limit I might hit which will necessitate using multiple?
For a single application you should only need 1 database.
But if you have multiple independent applications under the one hosting account, you should use a separate database for each application, although strictly speaking you wouldn’t have to.
There might be a total finite space limit for all your databases - whether you use just one or mutiple databases.
The time when you might want separate databases is where you have multiple sites sharing the space where they may not always be sharing in the future. Then one per site would be suitable.
Also not all hosting providers give you the access to set up all the different levels of security you may want for pieces of data and so then you’d need to split the data between databases based on who is allowed to access it.