Series: Como utilizar

Hi, everybody! In an FCE prep coursebook, I saw the following sentence:
"They made several series of Friends but the very first series were the best". We had to find a mistake in it, which I recognized as "was" instead of "were" because we should agree to "the very first". However, a st of mine called my attention to the word "series", which he had replaced by "episodes" as to make the sentence correct. I understand "series" as both a sing and plural word, but here I understood it as singular meaning "série de tv". Then, wouldn't it be possible to understand that they made several series with the cast of Friends? Oh, please, help me here! Many thanks in advance!