There can be a few ways to know your SBI policy number.
The most common way is to check the policy document. The policy number will be printed on the first page of the document.
If you have misplaced your policy document, you can contact the customer care of SBI Life Insurance. The customer care number is 1800 22 1111.
When you call the customer care, they will ask you for some basic details like your name, date of birth, etc. Once they have verified your identity, they will be able to provide you with your policy number.
alternately, you can also visit the nearest SBI branch and request for a duplicate policy document. The branch officials will be able to help you with this.
I hope this answers your question.