Modern is a constructed format and therefore adheres to the following constructed rules:
Minimum of sixty cards
No maximum deck size, as long as you can shuffle your deck unassisted
Up to fifteen cards in your sideboard, if used
With the exception of basic lands (Plains, Island, Swamp, Mountain, and Forest), your combined deck and sideboard may not contain more than four of any individual card, counted by its English card title equivalent.
Note: The five snow-covered lands from the Ice Age block—Snow-Covered Plains, Snow-Covered Island, Snow-Covered Swamp, Snow-Covered Mountain, and Snow-Covered Forest—are also basic lands. Snow-covered lands are only permitted in formats that allow expansions from the Ice Age block.
See here for the card sets permitted and cards banned in Modern tournaments: