Each pack has 14 cards that are put into the pool (10 common, 3 uncommon, 1 rare) giving everyone 42 cards to build their 40 card deck. About 40% of the deck will be land using the standard rules of thumb, leaving 24 cards out of 42 that are used in the deck, or 57% of the pool. To reach those same numbers the pool should be 63 cards, but that doesn't break evenly into packs, it is exactly 4.5 packs.
Both 4 and 5 packs are a good answer, for different reasons, it depends on what you want to focus on with the format:
- 5 packs allows for much more consistent decks. Drafting is unlike sealed because you get much more control over what's in your pool. 2 extra packs would let you stick to less colours and more consistent strategies, closer to what a constructed deck would look like.
- 4 packs still would force players to be creative in their deck construction. In drafts most people end up in 3 colours, 4 packs in a 60 card deck draft would be more likely to keep this happening. It will also keep cost a little lower if that's a concern.
It may also come down to what set(s) are being drafted if you need to round up or down, some sets are harder to build decks for (like all gold card conflux) vs sets which have clear colour distinctions and archetypes pushed by the card design (like Ixalan's 4 tribes).