How many MSSQL user CAL licenses should we buy?

We have a new installed sage 300 2017 with MSSQL server. I'm confused on how many MSSQL user CAL licenses we should buy. We have 5 staffs using the sage 300 (i.e. 5 Sage 300 logon accounts). But it seems that only one username needed in MSSQL server for Sage 300 to access MSSQL server directly.

My question is: should we buy one CAL license of MSSQL, or should we buy 5 CAL license of MSSQL? Why?