If you have Organization administrator access, follow the below steps to find list of users with Basic access
Step 1: Go to Organization Settings
Step 2: Click on “Users
Step 3: Choose Basic from the drop down as shown
Step 4: Once you choose Basic, you will be able to see the list of users with “Basic” access in Azure DevOps and also alternatively you can export the users to a excel file and filter it out to see who has basic access