I need help! Dragonball z boxset?

I think the book you’re talking about is 3-in1 Edition.
https://www.viz.com/read/manga/dragon-ball-3-in-1-edition/all.
These 3-in-1 Edition book includes both Dragon Ball and Dragon Ball Z. Z is the sequel of Dragon Ball.
If you have the first five 3-in-1 Edition books and you want to buy single volumes (each book 200 pages approx.) instead of 3-in-1, you need Vol. 16 of Dragon Ball and all the volumes of Dragon Ball Z (26 Volumes) to complete the series. Or, you can go with 3-in-1 Edition so you don’t get confused.

While the manga was all titled Dragon Ball in Japan, due to the popularity of the Dragon Ball Z anime in the west, Viz Media initially changed the title of the last 26 volumes of the manga to "Dragon Ball Z" to avoid confusion.

/r/manga Thread