Is it possible that there are more development years than accident years? What would that mean? How would the triangle look then? Would there be a triangle in the first place?
You can have as many as you like of either. Accident years are what you're monitoring (the number of rows in the triangle), and development years are how long you're monitoring each development year for (the number of columns in the triangle). For example, if you're only looking at one year, but it takes 10 years for claims from that year to finish developing, then you'd have a triangle with one row but 10 columns (not a triangle at all!).
