The size of manga pages isn't standardized. Some common sizes are around 180mm x 257mm or 176mm x 250mm. But it really depends on the publisher and the specific manga series.
It's hard to give an exact page count for the Akira manga as it can differ based on the version. Some editions might have more or fewer pages due to formatting and publishing factors. Usually, though, it's a substantial amount.
The Naruto manga has a substantial number of pages. Different collections and printings might have slight variations, but it's a massive work that's kept fans engaged for a long time. Estimates put it at well over 10,000 pages.
Twenty-one such books would have about 3270 pages.
This is because the total number of pages in each book will vary according to different authors, different publishing houses, different typography methods, and other factors, but there will usually be a certain average page range. And if a book has 196 pages, then 21 of these books will have about
3270/21 = 154 pages/book