Update add-prefix-allowlist.mdx (#26913)

Adding approximate limits per https://wiki.cfdata.org/pages/viewpage.action?pageId=375394487

Author: stechedo

src/content/docs/ddos-protection/advanced-ddos-systems/how-to/add-prefix-allowlist.mdx

@@ -22,4 +22,6 @@ To add an IP address or prefix to the Advanced DDoS Protection [allowlist](/ddos
 	5. To exclude the current prefix from the allowlist instead of including it, uncheck the **Enabled** checkbox. 6. Select **Add**.
 </Steps>
 
+Allowlists support approximately 200 IP addresses in a single expression for a rule.
+
 <Render file="allowlist-ip-spoofing" product="ddos-protection" />