How many users can use udemy account at a time?

Yes you can grant access to your udemy account to multiple users but unfortunately it involves the risk of getting your udemy account banned.

To share your accounts with friends, you’ll need a Udemy.com business account that lets you add a maximum of 5 users . Sharing your udemy account with friends or family can help them learn new skills or earn money from their content.

How to Sign up and Log in With the Udemy Mobile App

How to Change Your Udemy Account’s Email Address