This is a little difficult to predict without knowing more about your chosen business. However, allow us to share some very interesting data with you.

Over a period of two years we used a hosting package that was very similar to our Premium Hosting package.

In the space provided we were able to host 6 domains, 5 were AddOns. The traffic to those domains was reasonable for their size until about 18 months in.

We started having space problems as one site required lots of new posts daily along with photos. Those photos we should have resized them and uploaded them through the cPanel instead of through the WP platform media upload. The differences between the two upload procedures is the Cpanel method  requires less storage space.

At about the same time we tried something new on that same site and traffic skyrocketed. There was such a drastic increase that we got very close to hitting our bandwidth limit.

The average WordPress installation along with the normal files, database and emails takes up considerable less than 100 Megabytes to start, so you can see that even our starter package, Premium Hosting is more than adequate for many people. In fact most will find it adequate for years.

In our packages we tell you what the maximum amount of AddOn and SubDomains, but to have your site(s) perform at their best you should not fill them to this maximum. If you remember, Microsoft many years ago stated that you should either clean up or replace your hard drive when your free space dropped to 25%.

This sounds like a good idea, but even with that, when our server hard drives drop to 35% available space we will automatically add more.