Pakistan is a Federal Republic of four provinces, and every Provincial Assembly elects the same number of senators
Background
Article 1 of the Constitution provides that Pakistan shall be a Federal Republic to be known as the Islamic Republic of Pakistan.
The territories of Pakistan comprise the Provinces of Balochistan, Khyber Pakhtunkhwa, Punjab and Sindh, together with the Islamabad Capital Territory.
The Constitution refers to the Islamabad Capital Territory as the Federal Capital.
Seats in the National Assembly are allocated to each Province on the basis of population in accordance with the last preceding census officially published.
Members to fill the general seats of the National Assembly are elected by direct and free vote in accordance with law.
In the Senate, by contrast, the same number of members is elected by the members of each Provincial Assembly, by proportional representation by means of the single transferable vote.
The Senate shall not be subject to dissolution, and the term of its members shall be six years, with retirements staggered.
In plain words
Khyber Pakhtunkhwa — One of the four provinces of Pakistan, in the north-west of the country.
Provincial Assembly — The elected legislature of a province, whose own members vote in the Senate election for that province.
Single transferable vote — A system in which electors rank candidates and surplus or unused votes pass on, so seats track the strength of each group of voters.