First off, it depends on what kind of flying you want to do. Remember a flight instructor certificate is not required to fly an aircraft, either personally or professionally. The problem comes in that most professional flight jobs, either under part 121 or 135 require over 1000 hours total flight time and in some cases over 1500 hours TT. For a budding pilot with no other sources of income, this becomes almost insurmountable without doing flight instruction, as you’re legal to do this with 250 hours TT, a CPL, INST-A, and a CFI. If you do go the instructor route, the ratings you get for a flight instructor certificate all depend on what you’re going to do as an instructor. Instructor pay is based upon 1) the number of ratings that you have and 2) your dual given time. Remember, the more ratings you have the more flexible your employment options are; CFIs with 152/172 time are a dime a dozen. MEIs command the greatest salaries, though some schools only pay top dollar for multi engine dual instruction hours. Many schools today won’t employ you without at least a CFII (welcome to the insanity of flying for a living!). As to AGIs or IGIs, it again depends on the size of the flight school and the number of ground courses being offered. You can also do this as an independent contractor, but you will also have to recruit clientele, this is work in and of itself. Large Part 141 flight schools may have dedicated ground instructors for ground school, or may simply require new hire CFIs to hold ground instructor ratings. Ground instructor ratings are relatively easy to obtain; some home study followed by a testing fee of $300 or so for both the FI and AGI will get you one. Typical ground instructor pay is lower, on average, than flight instructor pay, but it all depends on your ratings and experience.
If you’re interested in making money as a CFI, my advice is to specialize. As I said before a CFI with dual time in Cessnas is so common that it doesn’t command much in pay in and of itself. Someone who can do, say, multi-engine, tailwheel, basic and advanced aerobatic training, CSIP certified, type ratings, seaplane rated, mountain flight training, warbirds, helicopters, gyroplanes, etc. can command over six figures as a CFI. The downside is that obtaining these additional ratings will cost hundreds of thousands of dollars so entry becomes prohibitive.
Now as to the numbers of people who become pilots and then flight instructors vs come through the door with an interest in flying, I’m not sure but I’d venture a guess that around 75% won’t even solo. Most people take a general interest in learning to fly then quit after a few flights from 1) the cost and time required to train, 2) medical or other barriers which prevent them from pursuing flight training and 3) the difficulty of flight training - many people expect something similar to the standards of US Driver’s Education or an MSF basic rider course and become disenchanted with flying when they can’t learn in a weekend by reading a 40 page booklet with a few flights. In the United States there are only about 600,000 licensed Pilots in a country of 300 million. Doing the math this works out to be about 1 in 500 people hold a Pilot certificate. Pilots holding instrument ratings are around half of the total ie about 300,000. Commercial, and Airline Transport Pilots - professional people who fly for a living - are around 100,000 and there are about 100,000 active flight instructors in the US as well. You can check the FAA’s Airmen Statistics for more precise figures. So if we go by the numbers of students at a typical flight school, out of 100 new people off the street, on average, only 25 will become licensed pilots. Of those 25, 12 will get instrument ratings and 6 will go onto obtain commercial certificates and flight instructor ratings.