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 ?
The API limits are for Incoming calls, ie From 3rd party to SF.
There is no limit for outgoing callouts from SF to external system/endpoint except platform limitation of 100 callouts in 1 transaction and no callout after DML
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.