Mitigations refers to the measures and actions that can be taken in response to the techniques in the MITRE ATT&CK matrix. Each mitigation has a unique ID, name and description that provides clear understanding about them. For example, the image below shows one of the enterprise mitigations:
(Image Source: mitre.org)

Note: Since the page in the above image has a long content, only the top part is shown.
Mitigations are grouped into 3 for the matrices as in other MITRE ATT&CK components:
(Image Source: mitre.org)

Note: Since the page in the above image has a long content, only the top part is shown.
The number of enterprise mitigations at the time of preparation of this training is as follows:
Mitigations: 43
You can check the below link to see the updated number:
Enterprise Mitigations: https://attack.mitre.org/mitigations/enterprise/
(Image Source: mitre.org)