I must confess is the 1st time i've heard that same account is used by the multiple people (except for Administratrive accounts) also because I'm not sure Microsoft would be happy with this model due to the CALs implications, not to mention the fact that this defeats the whole purpose of data security, permissions and overall you loose any sense of tracking of data managed in SharePoint.
Without going further into that topic, there is NO actual limit into how many simultaneous login-s of the same account are performed, but again, if you experience weird behaviors while editing one page, check in/out documents, etc. then you should think about that 1st hand.