Learn Arabic (Modern Standard): ما رقم الغرفة؟ رقم الغرفة واحد. - What is the room number? The room number is one.

ما is used to mean what in sentences without a verb, or right before a noun. Because What is the room number? does not contain a verb in Arabic, we must use ما. You would use ماذا if what came before a verb, like in What are you studying?

Why doesn't the word رقم have the Arabic word for the before it?

This phrase is a possessive structure, literally translating to number of the room. In Arabic, the first word in this kind of relationship never takes the prefix for the (ال). The the on the second word, الغرفة, is enough to make the entire phrase specific.

Where is the word for is in this sentence?

Arabic sentences do not use a present tense word for is or are. By simply placing the subject (رقم الغرفة) next to the number (واحد), the is is naturally understood.

Why doesn't the number واحد match the feminine gender of the word room (الغرفة)?

Here, we are not counting how many rooms there are (which would be one room). Instead, we are just naming the bare digit itself, like reading out a code or a phone number. When naming a bare digit independently, Arabic uses the default masculine form, واحد.