Populating a Junction Table in sqlite

Im a beginner in sqlite and I want to use sqlite in a database where I have three tables one as a junction table for the two tables. The two tables are named users and stocks table. I know that the user id in the users table will auto increment itself since it made it a primary key. Now in the stocks table I want it to increment it but it must start at 1 again if the stock is connected to another user which can be identified in the junction table.

Here is my schema enter image description here

And this is my idea on how I want my junction table to look like

enter image description here