As a continental, I would normally use serie to describe a single set, and series to describe multiple sets:
- I own a BWM 1 Serie, but I own a collection of 5 Series
- My favourite TV serie is The Simpsons, but I also like other series
In English, series is normally singular (like in the Latin etymon seriēs), e.g. BMW 1 Series.
a :a number of things or events of the same class coming one after another in spatial or temporal succession
// a concert series
// the hall opened into a series of small rooms
b :a set of regularly presented television programs each of which is complete in itself
However, I have also come across serie for the singular, e.g. in this JuPyteR Notebook on computing on tensors using FHE (1st sentence).
Is it ok to use serie? Are there specific contexts in which it is permissible?