[ask] Concurent User for OpenCart

Dear Seniors,

I want to know about dedicated hosting server specification and its correlation with number of concurrent users online together.

The thing I want to make is a multi online store. I mean that there are many store inside my website.

I have some options of hosting server, like :
256MB RAM 10GB Disk and 512MB RAM 20GB Disk

I wonder about the estimated number of concurrent users that online together for each options, and about need of storage size for my project.

Thank you and best regards,
achanon