I've heard someone saying"there is still 7 seats left". Shouldn't the sentence be "there are still 7 seats left".?
It should be "There are seven seats left", you're right. However, you'll hear "There is..." or more commonly "There's..." at the beginning at a lot of sentences because a) a lot of native speakers don't know the rule b) a lot of native speakers just ignore the rule and c) a lot of people start a sentence without thinking how it will continue.