Learn Farsi: برای کامل کردن این اتاقک، به صندلی‌های بیشتری نیاز داریم. - We need more chairs to complete this small room.

It uses the noun otāq (room) plus the diminutive suffix -ak, making otāqak (small room, booth, or cabin). The suffix -ak is often used to show that something is a smaller version of the original noun.

Could I use the suffix '-che' instead of '-ak' to mean small room?

While -che is also a diminutive suffix (like in bāghche for 'small garden'), suffixes usually stick to specific words by convention. For a room, otāqak is the natural, fixed word Farsi speakers use, while otāqche would sound unnatural.

Why is there a be before sandali-hā-ye?

The verb for 'to need' is niyāz dāshtan (literally 'to have need'). In Farsi, this verb always pairs with the preposition be (to) before the object that is needed. You are literally saying 'to more chairs need we have'.

Why does bishtar (more) have an -i at the end to make bishtari?

The -i at the end is the indefinite suffix. When attached to the adjective here, it shows we are talking about 'some more chairs' in a general, non-specific sense.

Why do we use the full dictionary form kāmel kardan after barāye?