Fortilink Switch scaling past 300

Hi Team,

I am looking at multiple customer scenarios where thee have more than 300 switches , or have an expansion plan in the future exceeding total of 300.

If I am interested  in proposing a fortilink fabric to maintain and operate them, what options do I have to increase capacity. 

As I understand an HA unit in A-A or A-P will not help here. 

Do we think of adding two HA pairs and then manage the whole show through Fortimanager ?

Your views would be appreciated.

Thanks !