Hi @Alexandre,
As we know, a List or a Library can contain 50,000 items with unique permissions, but the recommended limit is 5,000. If you apply unique permissions on a folder, and the documents in this folder have same permissions as parent folder, SharePoint will treat as one unique permission. So for your example, you have 2 items have unique permissions (Opportunity1 and Opportunity2), you can define 4'998 items with custom permissions.
Reference (from "Resource throttles and limits" section):
Manage large lists and libraries - Microsoft Support
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.