Series can be either singular or plural, depending on context. For example, you might write, “All of those television series are very good, but this series is my favorite.” Dictionaries do list a plural of series—serieses—but instances of this form from this century are extremely rare.
Harman International said Tuesday that its JBL Studio 1 loudspeaker series is now available. [Dealerscope]
Aired in late 2009, this unmourned five-part series starred Tamzin Outhwaite as Rebecca Flint, a detective attempting to solve crimes by looking at images sent to an astrophysicist from the future. [Telegraph]
One of these series contained not a single depiction of tobacco during the three months of the study. [Los Angeles Times]
The network plans to focus more on developing scripted shows, launching at least two more series by the end of the year. [Columbus Dispatch]