Maximum number of ActiveSync Devices

ActiveSync will only allow a default of 10 devices per mailbox.

To increase it to 50 devices, use this command in Exchange Management Shell

Set-Throttlingpolicy “Identity Name” -EasMaxDevices 50 -EasMaxConcurrency 50

http://www.msexchange.org/kbase/ExchangeServerTips/ExchangeServer2010/Mobility/HowtoincreasethemaximumnumberofActiveSyncDevicesforaUser.html