That depends on your applications needs. You first design your storage pool based on the applications that will be services from that pool. Then create the number TDEV's for each application according to their storage needs. You should read the Best Practices for Virtual Provisioning documents located on PowerLink. One way to navigate to this document is as follows: From the home page of Powerlink select Support > Technical Documentation and Advisories > Hardware/Platforms Documentation > Symmetrix DMX Family > Symmetrix DMX-3 > Technical Notes/Troubleshooting. You can also select the following hyperlink:  Best Practices for Fast, Simple Capacity Allocation with EMC Symmetrix Virtual Provisioning Technical Notes P/N 300-006-718           Forum Reminder: If you find any of the replies to your question as helpful, please be so kind as to mark replies as being "helpful" or "answered". This helps us track our success with the Forum.
Any count or limt for the creation of Tdevs.
how many TDEV’s we should create from the thin pools