What is the largest number of customer records that civicrm supports?
See this question: Number of contacts vs. hosting space required

Here is a thorough answer to that question: https://civicrm.stackexchange.com/a/3289/105.

The short version: there is no formal limit to how many contacts CiviCRM can store.  It really comes down to the amount and type of hardware you have supporting the software.  For a record count of 5-10 million, you would definitely need powerful hardware.