If I had only one day left to live, I would live it in my tenth-grade math class, because it would seem like infinity. If numbers do the same to you, fear not: you can master them by knowing the rules.
Plurals. Spelled-out numbers form their plurals as other nouns do.
Comma between digits. In most numerals of one thousand or more, commas are used between groups of three digits, counting from the right. However, in scientific writing, commas are often omitted in four-digit numbers. No commas are used in page numbers, addresses, and years (though years of five digits or more do include the comma).
Space between digits. In the International System of Units, half-spaces rather than commas are used to mark off groups of three digits, both to the left and to the right of the decimal point. In numbers of only four digits either to the left or the right of the decimal point, no space is used (except in table columns with numbers having five or more digits). This system is far more common in Europe than in the United States.
Source: Chicago Manual of Style