How do the Ausgleichsmandate in Germany's Bundestag election work?
I'm aware of How does the German Bundestag election system work? but it seems rather broad and none of the answers go into detail about the Ausgleichsmandate (or leveling seats as Wikipedia translates it). The English Wikipedia page doesn't explain it in much detail either:
In February 2013, following a decision of the Federal Constitutional Court which demanded a reform of the electoral law for proportional representation, Germany added a provision to create national leveling seats as needed in a case of negative vote weight occurring in its mixed member proportional system, in addition to the traditional leveling seats that already existed in many state elections.
I heard in a recent podcast that the Ausgleichsmandate can lead to (many?) more seats in the Bundestag with a default number of seats of 598 but estimates as high as having 900 seats. Perhaps not coincidentally, Philipp's answer to the other question mentions 299 and 598 which adds up to almost 900. That was two years ago even though the podcast mentioned the number changes over the year. The podcast spoke of calculations, suggesting that the 900 estimate they cited wasn't fixed.
So my question is: how does the Ausgleichsmandate in the Bundestag election work? Is the final number of seats known before the election starts or not? And by how much can the final number of seats vary depending on how people vote?