“ A number of the "stars" listed below may actually be two or more companions orbiting too closely for our telescopes to distinguish, each star possibly being massive in itself but not necessarily "supermassive" to either be on this list, or near the top of it. “

“ More globally, statistics on stellar populations seem to indicate that the upper mass limit is in the 120-solar-mass range,[1] so any mass estimate above this range is suspect. “

There are good theoretical reasons why a star shouldn’t normally get as big as the ones on the top of the list. Long story short: they’d very quickly shed mass due to their intense luminosity. Some of them might even be boiling with bubbles of pure radiation.

https://en.wikipedia.org/wiki/Eddington_luminosity

Beyond that, there’s also the possibility of pair-instability supernova, which might cause the most massive stars to literally disintegrate.