@Girish: Are you trying to call a .Net REST API from Salesforce probably 200-250 in same transaction? If so, the first suggestion would be to optimize .NET api to be able to handle bulk requests for e.g. 50-100 requests in same call. It will not only help you achieve it within Salesforce, but also improve performance as there would be less network usage. It's always a best practice to bulkify web service methods.
Alternatively, there would be following approaches (not optimal)
- As mentioned by other members, use batch apex to be able to surpass governor limit of 100 API calls in a transaction
However, I would still recommend bulkifying your .Net web service to handle more requests/ records in single invocation.