so buckle up. The main story of 'Tower of God' is divided into parts—Season 1, 2, 3, and the currently ongoing Season 4. The manhwa volumes you can buy collect chapters, not full story arcs, so the count gets messy.
To finish what's currently published, you're looking at the original webtoon chapters, not just the print volumes. For the entire story SIU has released so far? You need to follow the webtoon serialization, which is well over 500 chapters deep. The volumes are great for collectors, but they lag way behind the official English translation online. Honestly, chasing a 'finished' count with the books is a fool's errand; the story is still being told. The best you can do is catch up to the latest free chapter and then join the rest of us in agonizing weekly waits.
If you're asking about a complete, beginning-to-end book collection, that doesn't exist yet. The story is a massive, sprawling epic, and SIU has hinted at it going on for a long, long time. So, zero books are needed to 'finish' it, because it's not finished. You need patience, a good internet connection, and maybe a therapy pet for when your favorite character gets thrown into another impossible test.