SharePoint 2010 takes the first 3 property restrictions into account irrespective of the number of property restrictions specified in the search query

Using the Enterprise Search keyword syntax in SharePoint Server 2010, you can build a search query that uses multiple property restrictions to narrow the search focus based on the specified condition. However, SharePoint will only take the first 3 property restrictions into account irrespective of the number of property restrictions specified in the search query.

Currently, this is a known behavior. So, the next time when you build a search query, include 3 or less number of property restrictions using the search keyword syntax.

References

• Property Restriction Keyword Queries (TechNet)
• Using Operators in Keyword Queries  (TechNet)
• Property Restriction Keyword Queries (MSDN)
• Query.RowLimit Property (MSDN)

This does not state any kind of limitation on the number of property restrictions that can be present in a search query other than that the query cannot  exceed 2048 characters.