IEnumerable<T> is usually any collection of data. Array, List, HashSet, Stack, Queue, … you name it, they all implement
IEnumerable allowing code to easily
foreach over the entire collection.
If you have a single object, you’ll have to put it into a list. In fact I recommend not putting a BrepFace directly into a collection, as a BrepFace is always part of a Brep. Ideally you’d duplicate the face, creating a brand new brep:
Brep face = originalBrep.Faces.DuplicateFace();
List<GeometryBase> collection = new List<GeometryBase>();