The usefulness of that is limited, since you can only socket three items per character at Larzuk (one per difficulty as a quest reward), and it's more common for items to need less than max sockets to reach their true potential: 3 socket armor is much more valuable than four socket armor, because most of the good armor rune words use 3 sockets, and four sockets is much easier to find.

He'll give it the maximum number of sockets it can have, which depends on item type and item level (which is hidden, but can be inferred if you know what monster dropped the item). Larzuk is occasionally, if infrequently, a good choice for socketing a white item. The Horodric Cube recipes only work for non-superior items, which makes superior items with less than max sockets valuable, if they have the correct number for a popular item. There are two main ways to socket white items: the Horodric cube, and Larzuk's quest reward. You cannot add sockets to or remove sockets from an item that already has sockets, but you can add sockets to unsocketed items.

Diablo 2 cube recipes adding sockets