What does "numbers" mean here?

I see this paragraph in The life of Samuel Johnson

About this time he made one other effort to emancipate himself from the drudgery of authourship. He applied to Dr. Adams, to consult Dr. Smalbroke of the Commons, whether a person might be permitted to practice as an advocate there, without a doctor's degree in Civil Law. 'I am (said he) a total stranger to these studies; but whatever is a profession, and maintains numbers, must be within the reach of common abilities, and some degree of industry.' Dr. Adams was much pleased with Johnson's design to employ his talents in that manner, being confident he would have attained to great eminence.

I am not certain of the meaning of numbers in this context. Does it mean quantity (1, 2, 3,...) or instruction? Whatever, I do not understand what Johnson means in this sentence. Can you explain for me?

"Whatever is a profession" seems to be used in the sense of "any activity that comprises a profession".

"...and maintains numbers" seems to be used in the sense of "and (any profession) which is capable of establishing and reproducing an appreciable number of practising members".

What the author is asserting is his view that any profession is within the grasp of the person with average ability and a measure of hard work.

So "numbers" in this context means "an appreciable quantity of practising members".