What are the main ressources HAProxy needs handling millions of requests per hour? What hardware is recommended (CPU, Cored, Ram...)?
If millions of requests per hour (~300 rps), then entry-level servers will be fine, maybe single Intel Xeon + 8GB RAM.
If it's an API-balancing server, Gigabit Ethernet is fine. If it's a file download server with ~300rps, maybe choose 10Gbps network card.