There are various numeric limits, imposed either in the OS or in the specific shadow copy provider. Here is a table that summarizes all the numbers.
Note that the limit below is per volume, not per system.
Maximum number of shadow copies per volume
Windows XP
Windows Server 2003
Provider
Shadow type
The in-box provider
(ships with Windows)
Writers, non-persistent (used for backup)
1
512
No writers, non-persistent
n/a
Writers, persistent
No writers, persistent
Shadow Copies for Shared Folders
(no writers, non-persistent)
64
Third-party Hardware providers
Provider-specific (No max limit imposed by VSS)