Passage: Indeed, on 21 January 1951 the Castelo do Bode Power Station was officially inaugurated, the first of several large scale hydroelectric power plants that the National Electrification Law established in its project to supply electric power to the large consumption centres, such as Lisbon and Porto. From that moment on, the Tejo Power Station became a reserve plant, limiting its operation to years of drought or complicated water situations, and as a support system for the national electric grid. However, it continued to produce power, although operating only one turbo set and two high pressure boilers.

Question: How many boilers were operating after it had undergone limiting circumstances?
two