Well let's work this through.
In larger environements, the rule of thumb has always been to keep the number of vms per datastore to between 8-12, keeping in mind that this is a gross guide and assumes light/med-light duty vms.
Since the asked about environment is 8-10 VMs total, then really unless the disk is quite weak or the SQL server is especially demanding, a single datastore would pbly suffice.
Having said that - I usually like to build as though the system will grow - so in that case and given there is a sql server, then yes as I said my instinct would be to build prbly 3 datastores to start.