If you look at the TOS of the company, it is mentioned in the "ACCEPTABLE USE POLICY" that:
Using a shared account as a backup/storage device is not permitted, with the exception of one cPanel backup of the same account.
You agrees that you will not use excessive/High amounts of Resource usage (such as, CPU time, Daily cpu usage average, memory usage, network resources, etc). However we will give an option of paying extra for a Semi Dedicated or Dedicated server (It depends on your resources usage). We have the right to determine the high server usage level.
...is in itself an inherent contradiction. You cannot determine the thresholds yourself unless you start using the service. Hosting 50 domains on one account would surely cause a problem because each website will have its own share of traffic, taxing the server affecting other users on that particular shared hosting server leading to suspension without notice. Note: We have the right to determine the high server usage level
I would advise to steer clear of unlimited plans or carefully read their respective TOS and check for policies on fair use/suspension. Hosting companies usually have a threshold of allowing load to exceed for about 30-60 seconds, failure to stay within the limit leads to uninformed suspension and this holds true for quite a few renowned and big hosting companies out there. You wouldn't want 50 websites suspended altogether causing a bad impression on your visitors.
It may be feasible in the short run but might cause problems once your online presence starts to grow.
It also depends on what type of websites are being hosted. A single online store may equal 30 websites falling in XYZ category, in terms of load/resource usage.
Limited plans however would restrict web space, bandwidth, number of websites hosted per account to give you an idea with what you have to work with and peace of mind.