Graffito is the singular form, and graffiti is the plural. The words come from Italian, where this singular and plural pattern is standard.
In careful usage, you would say There was one graffito on the wall and There were several graffiti on the train. This makes the number clear: one mark is a graffito, more than one are graffiti.
In modern English, however, graffiti is often treated as a mass noun. That means people use it the way they use words like furniture or art. For example, someone might say The subway was covered in graffiti, even if they are talking about many separate markings.
- Singular: one graffito
- Plural: several graffiti
- Common modern use: graffiti as a general word for street writing or street art
So, if you want the traditional grammar point, use graffito for one and graffiti for more than one. If you hear graffiti used more generally, that is also common in everyday English.