Hello,
This is the limitation of the anti-phishing policy, you can only add up-to 60 users as of now. As a product feedback you can share your experience over the link below where i can see already the concern has been raised to increase the number of users count.
Please check the below mentioned link:
https://office365.uservoice.com/forums/289138-office-365-security-compliance/suggestions/35706682-remove-limit-of-60-users-for-new-anti-phishing-pol
You can certainly vote for the same, so that product team will look into it and will take your suggestions.
Cheers,
Saurabh
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