How many Force10 “port channels” are needed for a VMware port group

I'm not familiar with your switch but one gotcha on link aggregation is that it doesn't support LACP auto-negotiation so you have to configure the bonded ports statically on the switch. On a Cisco this is done by setting "no channel-protocol lacp" and "channel-group # mode on" rather than "channel-group # mode active" which seems to be an old style EtherChannel.

If I understand your topology correctly, there is no need to connect your 2960-S switches together at the access layer. While it can vary based on unique requirements, it is not a Cisco or Dell best practice to cross-connect access-layer switches.

And as long as you dont cross connect at the access layer, spanning tree protocol will not constrain any of the links in your topology.

While it would be a preferred design to connect each 2960 to each network core, this is not necessary. Because your servers are dual-homed to redundant 2960's, if one of your 2960 or S55 switches were to fail, the backup link should take over subject to the timers your fail-over method uses, so if this is acceptable, you are all set. If you did choose to add a redundant connection so that each 2960 has a connection to each S55, that could be configured as a link aggregation group since the S55's are stacked.

You are correct that you cant use standard lacp to create active/active links from your server unless you stack your 2960's; however many server nic's and hypervisor solutions now have nic load balancing options that are completely switch agnostic. If you are using server virtualization, you can most likely turn this feature on. If not, check with your nic vendor or in your advanced nic settings to see if this feature is available on your platform.