I haven't seen a tank shortage on Cactuar, unless you mean and EXPERIENCED tank shortage and honestly that depends on the duty. There are tons of sprout/returner tanks out there but you mostly see them in the lower end duties. I've seen quite a few in Expert and Alliance but for the most part, if the skill level falls through most of the time the party/alliance tries to compensate.

Tank is a leader role. A basic knowledge of the map/enemy groupings/and enmity skills (including what skills you use to actually pull *eyes tanks that use voke/auto attack to pull*) is expected and almost a zero-tolerance attitude if the tank can't play to that expectation. Many many people on this very forum have stated that they'll vote kick tanks that don't wall-to-wall pull or otherwise perform to the community tank stereotype of 'go big or go home'.

That's a lot to ask from a new player. Out of all the jobs, I think Tanking is the most thankless of all and it can be the most stressful because of the added weight of expectation and the fact that bad tanking is very readily apparent while mistakes on other roles are not quite as visible and often more forgiveable.