It's not directly possible. A database will always restore with the same number of files as it had when it was backed up.
Once you've restored you can move objects between the filegroups, or do a shrinkfile with empty if all the files are in the same filegroup.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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