A vast number is an incredibly large quantity, and there are many words that can be used to describe it. The term 'ARMY' is a four-letter word that can be used to refer to a vast number of people or things. A 'MYRIAD' is a six-letter word that means an extremely large number, often too large to count. A 'jillion' is a seven-letter word that is used to describe an unspecified large number. A 'googolplex' is a ten-letter word that refers to a number that is so large it is impossible to count. A 'HORDE' is a five-letter word that is used to describe a large group of people or things. Finally, a 'SLEW' is a four-letter word that is used to describe a large number of people or things.