Question

How is the number of electors each state receives in the electoral collage decided?

Answer 1

Conventionally, each state receives in the electoral collage THE NUMBER OF ELECTOR THAT IS EQUAL  TO THE TOTAL NUMBER OF SENATORS AND REPRESENTATIVES IT HAS.
An exception is Washington DC, which always has the same number of electors as that of the state that has the least population of people in US. 

Answer 2

Correct answer:  The number of electors each state gets equals the total number of members it has in Congress.

Explanation/detail:

Here's what the National Archives says concerning how Electoral College delegates are assigned: Electoral votes are allocated among the states based on the Census. Every state is allocated a number of votes equal to the number of senators and representatives in its U.S. Congressional delegation—two votes for its senators in the U.S. Senate plus a number of votes equal to the number of its members in the U. S. House of Representatives.

So the number of electoral votes each state gets (of the 538 total electoral votes) is recalculated every ten years, based on the most recent US Census data.