I only have one hosting account right now, but that's because I only have one website right now, in a bit of disrepair. However, back when I had multiple sites up, I had multiple hosting accounts.
I think the industry standard practice is to have one web hosting account for each site that you own, as that seems to be rather intuitive and less cluttered. It's also helpful when testing your site, as different sites may need different backend settings and configurations. There's less risk of fixing one site at the breakage of another. Finally, having separate accounts for each web design client makes it easier to hand over the proverbial reins when the design is done and it's time for the new webmaster to take over.
I like the idea of hosting multiple websites on one account, however. That could save you a lot of money over time if you have multiple small sites. The idea would be to have the sites in a separate folder for each one, and then point each domain to each folder. It's a creative idea that could pay off over the long term, especially if you're just getting started as a webmaster and don't know what will work. Just make a folder and domain for each prototype idea, test and go. Might be a good idea for a web design portfolio or software developer site as well.