The verb 'hay' is a special form of the verb 'haber'.
It means "there is/there are" in English.
There is no difference between the singular and plural form. This means that for the two sentences:
- There is one cat.
- There are 2 cats.
They are both translated like this:
- Hay un gato.
- Hay 2 gatos.
Yes, it's really that easy!
Don't get this verb confused with SER or ESTAR though!
If you just want to indicate that something/someone "exists", then you can use HAY.
If you say "Son dos gatos." It means "They are 2 cats." You are identifying what they are - it's a little different then just pointing out that there exist 2 cats in a specific area.