There is only one interrogative pronoun for quantity—колко—and it doesn't have any gender or number forms. It is used before plural nouns to ask about their quantity (then it is translated as how much/how many), and before an adjective or adverb to ask about the extent, degree, age, etc.
There exists a singular interrogative pronoun kolko used for quantity; it doesn't change based on gender or number. It is employed before plural nouns to ask about their amount (translated as how much/how many), and before adjectives or adverbs to inquire about extent, degree, age, etc.