With $10$ nodes, there are $4.2 × 10^{18}$ possible directed acyclic graphs. With $20$ nodes, there are $2.4 × 10^{72}$.
The number of DAGs follows sequence A003024. I haven't verified it myself, but https://sequencedb.net/s/A003024 lists the number when $n=20$ (note the sequence on that site starts at $n=0$) as
2_344_880_451_051_088_988_152_559_855_229_099_188_899_081_192_234_291_298_795_803_236_068_491_263
I know it doesn't change anything, and it is still $10^{72}$, but I thought I'd submit an issue for awareness.
The book states
The number of DAGs follows sequence A003024. I haven't verified it myself, but https://sequencedb.net/s/A003024 lists the number when $n=20$ (note the sequence on that site starts at $n=0$) as 2_344_880_451_051_088_988_152_559_855_229_099_188_899_081_192_234_291_298_795_803_236_068_491_263
I know it doesn't change anything, and it is still $10^{72}$, but I thought I'd submit an issue for awareness.