Two readings on the same character.
几 (3rd tone) = 'how many / a few' - by far the most common, used in everyday questions and compounds like
几个,
几点,
几岁.
几 (1st tone) = 'small table' / 'almost', a much narrower set:
茶几,
几乎,
几率. When in doubt, read jǐ.