How many adults can we have on our tenancy?

We are moving in to a new private rent property at the end of the month, after we had cleared all of thecredit checks we ( me and my partner) decided to ask our friends ( also a couple) to move in with us so we could half rent ect..now here's my problem, one guy at the Estate agents told us only me and my Partner could go on the Agreement because the landlord would need a different licence if we were all to be on the agreement, when we went for a second viewing the lady told us that all adults living in the property must be on the agreement and told us we could do it ( plus gave us forms to add the other couple on the agreement), anyone actually know what the maximum amount of people we can have on a tenancy ?