Learn English – Correct usage of USD

In the first sentence, If you are used to means 'If you are familiar with'. In the second, If you used to use means 'If you were in the habit of using in the past'.

You are being followed by zombies. You pick them off one at a time (shooting them in the head, of course) until there are no zombies left. Can you count the dead zombies?

Of course you can. They are countable. So the word to describe them is number.

A small number of zombies were chasing me. They were part of a large number of zombies that were detected in the city. (number + are)

Applesauce is oozing towards you, threatening to engulf you. To do so, it would need to be a very large amount of applesauce. Can you count the amount of applesauce that is threatening you?

No, you can't. Applesauce is uncountable.

A large amount of applesauce was on the ground nearby. (amount + is)

Likewise (but be careful when number is the subject; A definite number as subject is singular, but if it is an indefinite number, it's plural, and the verb reflects the plural. Check your determiner.)

A significant number of purchases were detected. One was for shoes, three purchases were for coats, and two were for umbrellas. The total number was six.