Well I’ve seen RT instance with 1,5M of tickets, thousands of users, hundreds of queues, hundreds of Custom fields and hundreds of groups. It was performing quite well but needed some tuning (mostly DB but also some queries). The complexity grows as cube, not linear - if you add a queue, the performance is influenced also with other already defined things (groups, CFs, …). If you’ve got only the huge amount number of tickets, it should be fine. Only one dimension of the whole cube is being stressed.
Next it depends on the database engine, I’d prefer PostgreSQL for big instances rather than MySQL/MariaDB. MariaDB is better than MySQL now so if you need to stay in that world, you should consider moving to Maria. Also the hardware counts but AFAIK the software itself scales quite well.