Here I have summarized the comparison with a list of selected features between vSphere ESXi 5.0, 5.1 and 5.5. I have skipped some of the features for detailed overview on all the comparison factors please visit vmware's official website.
ESXi Virtual Machine Maximum
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Virtual CPUs per virtual machine | 32 | 64 | 64 |
| RAM per virtual machine | 1 TB | 1 TB | 1 TB |
| Virtual SCSI targets per virtual machine | 60 | 60 | 60 |
| Virtual Disks per virtual machine | 60 | 60 | 60 |
| Virtual NIC per virtual machine | 10 | 10 | 10 |
| Concurrent remote console connection per virtual machine | 40 | 40 | 40 |
| Virtual disk size | 2 TB | 2 TB | 62 TB |
| Video memory per virtual machine | 128 MB | 128 MB | 512 MB |
ESXi Host maximum
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Logical CPUs per host | 160 | 160 | 320 |
| Virtual machine per host | 512 | 512 | 512 |
| Virtual CPUS per host | 2048 | 2048 | 4096 |
| Virtual CPUs per core | 25 | 25 | 32 |
| FT Virtual Disks | 16 | 16 | 16 |
| RAM per FT virtual machine | 64 | 64 | 64 |
| FT virtual machine per host | 4 | 4 | 4 |
| FT virtual CPUs per virtual machine | 1 | 1 | 1 |
Memory Maximum
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| RAM per host | 2 TB | 2 TB | 4 TB |
| No of swap files per virtual machine | 1 | 1 | 1 |
| Swap file size | 1 TB | 1 TB | NA |
Storage Maximum
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Virtual Disks per host | 2048 | 2048 | 2048 |
| LUNs per server | 256 | 256 | 256 |
| Broadcom 1Gb iSCSI HBA initiator ports per server | 4 | 4 | 4 |
| Broadcom 10Gb iSCSI HBA initiator ports per server | 4 | 4 | 4 |
| No. of total paths on a server | 1024 | 1024 | 1024 |
| No. of paths to a LUN | 8 | 8 | 8 |
| Software iSCSI targets | 256 | 256 | 256 |
| LUNs per host | 256 | 256 | 256 |
| LUN size | NA | 64 TB | 64 TB |
| No. of paths to a LUN | 32 | 32 | 32 |
| No. of total paths on a server | 1024 | 1024 | 1024 |
| No. of HBAs of any type | 8 | 8 | 8 |
| HBA ports | 16 | 16 | 16 |
| Targets per HBA | 256 | 256 | 256 |
| Concurrent vMotion operations per datastore | 128 | 128 | 128 |
| Concurrent storage vMotion operations per datastore | 8 | 8 | 8 |
| Concurrent storage vMotion operations per host | 2 | 2 | 2 |
| Concurrent vMotion operations per host (1Gb/s network) | 4 | 4 | 4 |
| Concurrent vMotion operations per host (10Gb/s network) | 8 | 8 | 8 |
Cluster and resource pool maximums
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Hosts per cluster | 32 | 32 | 32 |
| Virtual machine per cluster | 3000 | 4000 | 4000 |
| Virtual machines per host | 512 | 512 | 512 |
| Resource pools per host | 1600 | 1600 | 1600 |
| Children per resource pool | 1024 | 1024 | 1024 |
| Resource pools per cluster | 1600 | 1600 | 1600 |
vCenter Server maximums
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Hosts per vCenter Server | 1000 | 1000 | 1000 |
| Powered‐on virtual machines per vCenter Server | 10000 | 10000 | 10000 |
| Registered virtual machines per vCenter Server | 1500 | 1500 | 1500 |
| Linked vCenter Servers | 10 | 10 | 10 |
| Hosts in linked vCenter Servers | 3000 | 3000 | 3000 |
| Concurrent vSphere Client connections to vCenter Server | 100 | 100 | 100 |
| Number of host per datacentre | 500 | 500 | 500 |
| Registered virtual machines in linked vCenter Servers | 50000 | 50000 | 50000 |
Storage DRS
| Items | Maximum | ||
| vSphere 5.0 | vSphere 5.1 | vSphere 5.5 | |
| Virtual disks per datastore cluster | 9000 | 9000 | 9000 |
| Datastores per datastore cluster | 32 | 32 | 32 |
| Datastore clusters per vCenter | 256 | 256 | 256 |
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