You don't need any special word to count something like tennis courts. To make your sentence correct, change the first comma to semicolon and delete the word "pieces." The number of tennis courts in the Tennis Club is growing; at the beginning of 2016, there were 13. You could also put a period after "growing" to make two independent sentences.

thank you, Melinda!

The word "growing" is generally used to describe a living thing (children are growing up, the bamboo is growing). Other things are "increasing." Also the word "pieces" is awkward in this sentence. I would use the word "courts" again.