I will use as an example a UK general election, which refers to the election of Members of Parliament to the House of Commons. A general election is made up of 650 individual elections [as there are 650 geographical areas, known as parliamentary constituencies] across England, Wales, Scotland and Northern Ireland.
It is "election" if we refer to the process or result of a general election of the country taken as a whole.
e.g. "Today is Election Day!"
"Who has won the election?"
It is "elections" when we are talking about all something pertaining to or affecting all 650 constituencies:
"Tomorrow, this country goes to the elections."
"The elections have seen poor turnouts across the country."
"There is a by-election tomorrow in Manchester Central."
"By-elections are being held in Corby, Cardiff South and Penarth."