For the below requirement i would do stats count only, But will not be able to show other details. I would do a join to just show the count.
http_status="500" | stats count by url|join host[http_status="500" | stats count(http_status) as Error_Count]|table server_name,client_address,url,http_status_description, http_method, http_version, user_agent, referrer,count,Total_Error_Count|where Total_Error_Count > 100
this will show you Total_Error_count same for each of the urls/ips.
Hope this gives you a hint, but it's not the correct condition for alert to see.
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