Welcome to the MongoDB Community @Rajitha_Hewabandula !
For self-hosted MongoDB deployments there is no limit on the number of users you can create, but typically you would not want to have 100s of admin users with direct access to a deployment. For a general reference for the MongoDB server please refer to MongoDB Limits and Thresholds.
MongoDB Atlas has some Organisation and Project Limits including 100 database users and 500 Atlas users per project, but if you have additional requirements you could contact support. For related discussion, please see The 100 User Limit in Atlas - #5 by Stennie.
Regards,
Stennie