Fifty contestants are competing for first prize this year.
What do you think the subject is?
competing
The verb is "are competing." Ask yourself, who "are competing" and you'll know what the subject is.
What do you think?
To calculate the total number of contestants competing for the first prize, you simply need to count the number of contestants given in the question. In this case, the question states that there are fifty contestants competing for the first prize.
So the answer is: There are fifty contestants competing for the first prize.