Per this website : https://learn.microsoft.com/en-us/defender-endpoint/configure-extension-file-exclusions-microsoft-defender-antivirus

You can only use a maximum of six wildcards per entry.

More :

• Environment variable usage is limited to machine variables and those applicable to processes running as an NT AUTHORITY\SYSTEM account.
• You can only use a maximum of six wildcards per entry.
• You cannot use a wildcard in place of a drive letter.
• An asterisk * in a folder exclusion stands in place for a single folder. Use multiple instances of \*\ to indicate multiple nested folders with unspecified names.