Learn Farsi: در دفتر چند همکار دارید؟ - How many colleagues do you have in the office?

In Farsi, whenever you use the question word chand (how many) or a specific number, the noun that follows it must always be singular. So, you are literally asking 'how many colleague'.

I previously learned that daftar means notebook. Can it also mean office?

Yes, daftar is a versatile word that means both notebook and office. When meaning office, it is often a shortened form of daftar-e kār, which literally means 'office of work'. The context here makes it clear that you aren't asking about colleagues inside a notebook.

Why doesn't the question word chand go at the very beginning of the sentence?

In Farsi, question words do not move to the front of the sentence like they do in English. Instead, they sit in the exact spot where the answer will go. Since the answer would be 'in the office I have [five] colleagues', the word chand replaces the number right before hamkār.

Why isn't the word for 'you' in this sentence?

The verb dārid ends with -id, which specifically means 'you' (either plural or formal singular). Because Farsi verbs change their endings based on the subject, the separate pronoun shomā (you) is optional and usually dropped.