Upto how many changes does track field history kept in salesforce?

Hi Pranav,

Hope this will help you.

You can select certain fields(up-to 20 fields) to track and display the field history in the History related list of an object. The field history data is retained for up to 18 months.

There is no limit for changes to record by Field History Tracking.

You can track the field history of custom objects and the following standard objects.

• Accounts
• Articles
• Assets
• Cases
• Contacts
• Contracts
• Contract line items
• Entitlements
• Leads
• Opportunities
• Orders
• Order Products
• Products
• Service Contracts
• Solutions

For more information please go to below mentioned link:-

https://help.salesforce.com/HTViewHelpDoc?id=tracking_field_history.htm

Thanks