You don't need to be concerned about it. In IGNOU, there is no such thing as an assignment number. You may have been perplexed by the term Assignment Code. IGNOU only requires an enrolment number.
When one talks about "assignment code," they are referring to the information on the top right side of your assignment question paper. It displays the year of your assignments (session), such as 2019-2020 or 2021-2022. It shows the validity of your assignment, such as which year it needs to appear in your TEE.
You must include the assignment code, as well as other details, on the first page of your IGNOU assignment.