Each EtherChannel can consist of up to eight compatibly configured Ethernet ports. All ports in each EtherChannel must be configured as either Layer 2 or Layer 3 ports.
The Cisco IOS Software Release 12.2(25)SEA supports up to forty-eight EtherChannels, but in earlier releases for Catalyst 3750 series switches, the number of EtherChannels was limited to twelve.
Hence, this problem is due to the limitation of the Cisco IOS software release that runs on the switch.
In order to configure more than twleve EtherChannels, Cisco IOS Software Release 12.2(25)SEA or later must be used. These software releases can be downloaded from Cisco Downloads.
Refer to Cisco Catalyst 3750 Series Switches Release Notes for more information.