I looked at the salesforce documentation.
Maximum simultaneous requests to URLs with the same host for a callout
request is 20
Simultaneous means starting at the same time while concurrent means running parallel (may not be started at same time.). Example your Sales-force org will have many concurrent sessions as most of users are using salesforce but it is highly unlikely that all of the have logged in at same instant/simultaneously.
Since the lifetime of a request is very short(Public sites) hence you can consider both concurrent and simultaneous as same.
From Sales-force Documentation:
The host is defined by the unique subdomain for the URL—for example,
www.mysite.com and extra.mysite.com are two different hosts. This
limit is calculated across all organizations that access the same
host. If this limit is exceeded, a CalloutException is thrown.
So If 20 other users across all salesforce organizations will access the same host at same point in time (Probably calculated in milliseconds), you will hit this salesforce limit.