If your father holds title in his name (rather than a partnership name or corporation), it is possible to seach records by county. I have access to public records information for Alameda and Contra Costa County, but if you're looking to search the whole state I would suggest contacting an attorney.
Keller Williams Realty
You can start by checking to see how they held title on the house they owned and lived in until they passed away. It may give clues on whether it was held in a trust, or corporation, etc.
To ask this question on a public forum like this will generate suggestions on how to find out the answer to your question, but will not give you the number of properties your parents owned in California