Unless something is different in your state there is no need to warn someone before sending them to collections, if your bill is behind you are fair game. Usually before someone takes you to small claims court they must first attempt to collect the debt and be able to demonstrate that.
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Legal Information is Not Legal Advice
My answer provides information about the law based on the limited information provided in the questions asked and is not intended to be legal advice. The law differs in each jurisdiction and may be interpreted or applied differently depending on the location or situation. I highly recommend that you consult with an attorney to discuss the specific details of your situation so you can get legal advice tailored to your specific circumstances. The information in my answer is for educational and information purposes only, and is not legal advice or legal opinions. The answer provided to the question asked does not constitute a lawyer-client relationship.
No, there is no magic number as to how many times (if any) a creditor must warn you before sending you to collections.
In other words, a creditor can send your account to collections without warning you at all. Your first knowledge of the action may well be when you hear from collections.