How often must Muslims perform hajj?

It is required for an able Muslim to perform hajj once in their lifetime; if you perform it multiple times, then that's a voluntary act of worship.

It was narrated from Ibn 'Abbaas that al-Aqra' ibn Haabis asked the Prophet (peace and blessings of Allaah be upon him): "O Messenger of Allaah, is Hajj every year or just once?" He said: "It is only once, and whoever does more than that, it is a voluntary (act of worship)."