If its in a subdirectory of the domain, then you shouldn't need to request a new API key.
To map all the client locations on one map, the easiest way to do this would be to store the different parts of the address (street, state, zip, etc) in a database, then add a recordset to the page that gets the different address columns. In the pro maps interface, on step 2, you can click on the show multiple checkbox and bind the values to the different columns in the recordset.