Please don't take these as definitive confirmations since these are just some statements I vaguely remember reading a long time ago, but I believe that it was mentioned in some old Q&A that each writer on the team was writing one companion each.
If I recall correctly there are 12(?) writers on the team which also coincides with the fact that the D&D 5e PHB has 12 classes.
Based on these two things I'll make the bold assumption that there'll be 12 (or at least around 12) companions, so to answer your question: yes, there'll be more companions added later on.
