I can read on licensing guide this section:
"Named CALs are assigned on a “named user” basis, meaning each user requires a separate User License; Named user licenses cannot be shared but an individual with a named user license may access the service through multiple devices."
Does the words in bold mean I can access concurrently? If I suppose to have 10 named user licenses can the User1 connect more than one time under the 10 licences? Ex. User1 (PC), User1 (Tablet), User1 (Smartphone), User2 (PC), User2 (Tablet), User3, User4, User5, User6 (Tablet), User6(PC)?
Yes that is correct.
Ok, another little question. If an user try to connect after the limit cannot it connect or does it steal a session of another concurrent user?
No. If there are no more concurrent users, then the user must wait until a user disconnects.
If using the web client, then sessions are automatically disconnected after a timeout.
But why do you ask about concurrent users and Business Central. With Business Central, no matter if you talk about On-Premise or SaaS, then your only option is named users. Any existing Dynamics NAV licenses needs to by migrated to BC named users (where you get 2 named for each concurrent users assigned to the license before October 2018).
Your only other option is to stay on NAV 2018. The date to buy new NAV 2018 licenses has already passed (March 31st 2019), so that's only if we talk about an existing NAV license.