I applied for medical exception for CA state now form 3853 is asking for a ECN number. Where do I find that number or if now I have to wait for something in the mail?

Covered California says

Once you have submitted a complete exemption application, Covered California has up to 30 calendar days to determine whether you qualify. We will mail you a notice with our decision. The notice could show that your exemption was:

• Approved;
• Denied; or
• Had missing or inconsistent information and what steps to take.

If it has been well more than 30 days, then I would call Covered California. Surely the letter from Covered California showing that your application was approved will have this number.