As far as we know, there is no limit to what they can absorb. As they get more massive their event horizon (the radius where even light can’t get away) gets bigger, but they never get “full”.
Depending on exactly what happens inside the black hole it’s either got an infinitely dense core, so it literally has no size and can hold as much mass as you like, or it’s just got an insanely dense core that’s so small that it behaves the same way and it doesn’t really make a difference.
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