This is quite a common error but this one had a little twist. I had created a new Exchange 2013 organization and the client required 7 mailbox data stores. I mounted the first 5 then when I tried to mount the 6th and 7th I was greeted with the following error.
The error cause was staring me straght in the face, if we read the section MAPIEXCEPTIONTOMANYMOUNTEDDATABSES we can see its complaining about the 6th database. The reason I was having this issue was that I had not put the license key in for Exchange 2013. Out of the box unlicensed Exchange 2013 will only allow 5 databases to be mounted concurrently, which is normal for the standard edition of Exchange 2013.
If you would like to mount more than 5 databases then you will need the enterprise edition and this edition is again limited to 50 databases per server.