I think it's 75 rounds for the 7.62x39mm ones, but that's for the RPK, not sure if the Type 81 has a different design.

100-rounders seem to usually be of the Beta C-Mag type:

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and conventional 100-rounders are a lot bigger than the one on this Type 81 (although it's kind of hard to see on this tiny pic, I can't access the website it's from for I assume "American website owners don't want to bother with GDPR so they just IP-blocked Europe" reasons, so all I have is a thumbnail that Google's cached, but you might be able to open it up properly):

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