The discovery of TON 618 have created a new black hole species (already fingerprinted by M87 or even IC1101 cores): the ultramassive black holes with masses greater than $10^{10}M_\odot$. As said in the previous answer, in classical settings, there is no upper limit of the mass of black holes (I am not so sure if you get a theory beyond General Relativity even in classical settings).
Maybe, one day we will learn that quantum gravity says something about that. Interestingly, any supermassive, stellar, intermediate and ultramassive black hole has a mass much much greater than Planck mass, about a microgram. The issue is that we think quantum gravity applies only to VERY MASSIVE TINY (very dense) objects, not to very massive only. Indeed, any person has a mass much greater than Planck mass, but it is not "concentrated". When you have concentrated mass in very tiny regions, we have no idea of how to handle quantum fluctuations and amplitudes excepting with superstring theory. Another related question, is if you can have black holes of any DENSITY. Again, as said, you need to consider quantum processes like Hawking radiation, ... However, there is a subtle point, called the transplanckian problem. In principle, as the black holes evaporates it gets smaller and smaller, such as at certain size the wavelength would be lesser than the Planck length. We have to expect for a definitive theory of quantum gravity before to answer the ultimate fate of black holes and thus, the destiny of both: black holes and the whole universe (even the spacetime could be metastable and provisional/transitional state).
How large can a black hole formed from the collapse of a massive star grow
in 1 Gyr? Suppose the black hole can grow as fast as it can. Suppose, by the moment, it satisfies the Eddington limit. Then, an exponential law follows up:
$$\dot{M}=kM=M/\tau$$
where $k=4\cdot 10^{-16}s^{-1}$ for a ten solar masses inicial mass function accordingtly to the Eddington limit. Then, as
$$M=M_0\exp(kt)$$
Plugin into this formula $M_0=10M_\odot$ and the value of k, you get that the maximum mass it yields is in the range of ultramassive BH, i.e., $M_f\sim 10^{10}M_\odot$ for a timescale about 1 Gyr (be aware, the numbers are tricky). Of course, transEddington limit is tricky, but there are some reasons to believe black holes bigger than $10^{10}M_\odot$ are unstable and eject material. Of course, in the absence of any other argument, the above argument does NOT provide an upper limit in principle. Only other considerations relativo to quasars and jets seem to apply. But the issue is a hot topic of debate in astrophysics.
By the other hand, the minimal (or tiniest) black hole mass is also a mystery. In macroscale, we have NOT found black holes tinier than 3-5 solar masses (stellar black holes). However, primordial black holes or microblack holes could made some bits of the dark matter hidden in clusters and other parts of the galaxies. Again, the only hint are inflationary ideas, astronomical measures and experimental bounds (recently, it has been analyzed the probability of dark matter being totally black holes, but some evidence seems to say that that is not the case: black holes can not be all the dark matter).
