grammatical number - A significant amount of zombies were detected in your city

Can anyone please help and tell me if this sentence is correct?

"A significant amount of zombies were detected in your city."

In my opinion it should be "A significant amount of zombies was detected in your city." because "amount" should be in agreement with "was" even though amount usually refers to something plural. Are both (was and were) acceptable in this case?

Edit1: Yes, "number" would be the correct one in this case. So allow me to ask one more thing.

"A significant amount of purchases were detected..."

Does the use of "number" apply in this case also? As purchase is not exactly a thing/person. And if no, is it correct to say "A significant amount of purchases were detected..."?

Edit2: So, please tell me what is the correct phrasing?

"A significant amount of purchases were detected..."

"A significant amount of purchases was detected..."

Or, since purchases are countable, is the use of "number" instead of "amount" correct here, as in:

"A significant number of purchases were detected..."

"A significant number of purchases was detected..."

Or would amount be correct since a purchase involves money and it refers to the amount of money?

Would greatly appreciate an answer, and if possible a detailed explanation.