I've been told its against the law, not to inform the County Sheriff's department if you plan to host a motorcycle rally where participants exceed 5,000.
Is this true? And where can I look up the law code on this?

How about asking the person who gave you this advice?

to quote our home page ....

"Welcome to the TDCAA Bulletin Boards. The discussions in these user forums are for the benefit of prosecutors and their staff members, although we welcome relevant and appropriate input from other members of the criminal justice and government lawyer community. These forums are NOT a source of legal advice for citizens. Call the State Bar of Texas (1-800-204-2222) for information on seeking legal advice."