“Cuantas carpetas el/ella tiene en su bolso/mochila/bulto”.
You can say backpack in many ways! Bolso is backpack, mochila is backpack and so it bulto! I suggest to use which ever you could pronounce :)
Also, spanish uses female and male pronouns “el is him”,”ella is her”, there is no they/them. The only way you would use “they/them” is when your referring to a group of people. Like: “Cuantas carpetas tienen ellos”, which translates to how many folders do they have, but it is not grammatically correct so you would have to refer to them as either a she or a he.