“几” indicates an approximate number.
When the speaker doesn’t know, doesn’t want to tell or can't tell the exact number, the approximate numbers can be used.
In this lesson, the approximate number "几" is introduced. "几" means several, which indicates an inexact number from two to nine. It can also used before "十 ten, 百 a hundred, 千 a thousand, etc., or after "ten."
“几 several” refers to an inexact number only from two to nine.
“十几” refers to an exact number between ten and twenty. “二十几” indicates an exact number between twenty and thirty
and so forth, such as 三十几、四十几……九十几, etc.
“几十” means scores of, which refers to an inexact number from ten to ninety. “几百” means hundreds of. It indicates an inexact number from one hundred to nine hundred. “几千” means thousands of, which indicates an inexact number from one thousand to nine thousand. “几万”, or ten thousands of, refers to an inexact number from ten thousand to ninety thousand.