In our current setup when one AP down rest of the client move to other AP, we need to move them manually or connect/disconnect. I can apply max-association on SSID but is there way I can do it automatically ?
The "final" decision to join which AP and/or which radio falls on the wireless client. If the wireless client wants to associate to the furthest AP then there's nothing you can do.
One way of minimizing this from happening is to ensure your wireless clients are running the latest drivers. On the AP-side, you could also try to disable the low-speed data rates (from 11 Mbps and below, make 18 Mbps as Mandatory and the rest are Supported).