How many mail accounts can I have?

So Im having an issue with Mail in my 2012 Macbook Pro.

Almost every time I use MacMail it either doesn't load properly, OR loads mail, but when i try to quit it all I get is the 'spinning wheel of doom' which requires that I force quite the application.

I spent 3 hours on the phone to AppleCare who had no idea, I then spent 3 hours at the local Genius Bar who had no idea, they did a clean install of the OS and all of the emails accounts but MacMail still continued to 'lock up'.

My local store has now had it for 14 days and tell me they don't know whats wrong with the machine. All they can think of is that I have too many email acounts on it. I have 7 in total.

I don't buy it, the machine worked perfectly with these 7 email accounts for almost 18 months previously without issue.

Also my iMac has 8 different email accounts, is on the same OS and works perfectly.

Even my old 4s iPhone and iPad 2 work without issue, although they only have 5 accounts on each of them.

Any Ideas?