I know that we can make callouts based on Salesforce edition and its licenses, is there any limitations on user level ? i.e How many API requests can an user make in 24 hour period ?
API calls are metered per organization, not per user. Assuming a user has the Api Enabled permission, they can theoretically make as many API calls as the organization has available, subject to other limits, such as the concurrent request limit. In small orgs, this is pretty to do, while in really large orgs, a user might need to exceed 100 API calls per second to max out the org's limits.