17; that was in 1918. They quickly increased the electoral threshold to 75% of a seat then. Then, in 1933 when there were 14 parties in parliament again, they increased it to 100%. After that, it has always hovered around 10 parties, and it's still one of the lowest electoral thresholds in the world; most countries have it at 5%-10% of the total vote.
If I count correctly, the 2003-2006 Tweede Kamer had 9 party fractions in it at the start and had 5 split-off groups, so there were 14 fractions in the end. Maybe there were more somewhere in the meantime?
But of course I don't know what xenobioticants was thinking about.