How many devices the maximum support of HA Group?

i think it is 8 (maximum number of devices in traffic group) since, you know, ha group for traffic group is introduced in 11.5.0.

HA group failover for traffic groups

Prior to this release, the HA group feature of the BIG-IP system calculated a single HA health score for per device, and if the score fell below a configured threshold, the system initiated the failover of all traffic groups on the device. With this release, you can configure a separate, unique HA group for each traffic group instance on a device, causing the BIG-IP software to calculate a separate health score for each traffic group instance on a device. The result is that the system can initiate failover for a specific traffic group according to the needs of the application traffic associated with that traffic group. For example, if you create traffic-group-2 containing the virtual IP address 192.168.20.10, and you create an HA group based on trunk health and assign it to the instance of traffic-group-2 on device Bigip_A, then that instance of traffic-group-2, if active for Bigip_A, fails over to another device in the device group whenever the number of links falls below the specified threshold.

Release Note: BIG-IP LTM and TMOS 11.5.0

http://support.f5.com/kb/en-us/products/big-ip_ltm/releasenotes/product/relnote-ltm-11-5-0.html