Master is what someone else may call a teacher. Not what a teacher calls themselves. (ego is a dangerous thing).
Master is not a title conferred or awarded to someone. It is an act of recognition from a master/teacher when students studies have matured enough that they can walk on their own path.
This does not even require they know an entire curriculum of forms or weapons of an art. Just that they have trained and done the work to gain the skills the art is meant to train. For example, Hong Junsheng trained under Chen Fake for many years and only did bare hand training, no weapons. Even though the curriculum at this time would have included the Chen style sword, Guan Dao, etc. He felt he needed to focus his training more and became a very famous high-level Master of the style. The martial arts are full of stories like these, but they tend to be fantastic legends in the past. So I will give another more modern example.
One of my gongfu brothers has trained with a renowned southern master for about 25 years now. While he has learned a few forms from Master Chan he has heavily worked on one for the past twenty years – truly mastering it. Apparently one day he went to the club and Master Chan told him to light incense and do the altar which was very unusual since it is normally the job of a younger student. Afterward, Master Chan told him he was finished his training, Master level. No weapons training, no special uniforms, no certificates and I have to say he is one of the best Gongfu Masters I have ever known.
Breadth is not as important as the depth of knowledge.
Belts are for holding up pants.