You can increase this limit by using Throttling policy.
First Create new throttling policy.
New-Throttlingpolicy "Increased ActiveSync Devices1" -EasMaxDevices
600 -EasMaxConcurrency 20
-EasMaxDevices 20 (Maximum mobile phone limit)
-EasMaxConcurrency (Maximum concurrent device that will sync.)
Then assign this policy to user.
Set-Mailbox username -ThrottlingPolicy "Increased ActiveSync Devices1"
Check if Policy applied to mailbox
Get-Mailbox username -ThrottlingPolicy | FL