"There are two chairs."
The verb あります/ ある, which roughly translates to "to exist", requires が (the subject marker). Like the English counterpart it always needs the subject. Most Japanese verbs are not like that but ある is special.
(if you know French, it's good to think of it the verb "avoir"/ to have )
が is a topic marker. Like は, it can indicate a subject of the sentence, and in common conversation が is often used to introduce a subject not already part of the conversation. It can also be used in place of the は to indicate a point of focus for your sentence, in English the same thing is typically done with tone.
For example, if you wanted to put emphasis on "homework" in "I finished five pages of homework" you would say "宿題が五枚終わりました。" It would also be grammatically correct to use the は particle, especially if the homework was already part of the conversation: "宿題は五枚終わりました。"