Are there limits to tracking fields history?

You can track upto 20 standard and custom fields per custom object.

To track field history for custom objects:

1. From Setup, click Create | Objects.

2. Click Edit next to the name of the custom object.

3. Select the Track Field History checkbox.

4. Click Save.

5. Click Set History Tracking in the Custom Fields & Relationships section. This section allows you to set a custom object’s history for both standard and custom fields.

6. Choose the fields you want tracked. You can select up to 20 standard and custom fields per object. You can’t track: Formula, roll-up summary, or auto-number fields Created By and Last Modified By

7. Click Save.Salesforce tracks history from this date and time forward. Changes made prior to this date and time are not included.

Look into this What is the best workaround for the 20 field history tracking cap?