How many stars does one create and how many times can stars be recycled into more stars?
It depends on the nebula and on the masses of the stars that are produced. The stars in a new population usually end up following a statistical distribution of masses called the stellar initial mass function (IMF). I'd recommend looking it up. There are different proposed IMFs, but they all share the property that you get more low-mass stars than high-mass ones (i.e. high mass stars are rare). It's still unclear whether the IMF is universal or whether it differs from place to place. But either way, it's a statistical thing. Obviously a given individual nebula cannot produce any really high-mass stars if there isn't enough raw material present to produce even one (even if the IMF predicts there should be some number of them at that mass).
This is a complex topic. I think that as long as the interstellar medium (the gaseous matter in between the stars in a galaxy) continues to be enriched by new raw materials from dying stars, then star formation can continue. But I do know that there are processes that can stop star formation in galaxies (which is called "quenching"). In particular, tidal/gravitational interactions between galaxies (collisions, mergers etc) can strip the gas out of the galaxies involved and therefore remove all of the raw materials necessary for continued star formation. (Particularly if all of the high-mass, short-lived stars are gone, and all that remains are low mass, long-lived ones that are never going to go supernova and aren't going to end their lives for a very very long time, longer than the present age of the universe). Such galaxies are sometimes referred to as being, "old, red, and dead", and almost all elliptical galaxies are like this. They have all stars and no gas.
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