There are 975 different items in the game. You can check your progress while playing by pressing
--> Misc --> Info.
If you are going for full completion on this game, and exploring as you go, this will probably come with natural progression. However, if you're specifically looking to bump up your item count, here are a few suggestions:
-Craft every recipe you find, even if the item isn't an upgrade, to guarantee credit for that item.
-Buy at least 1 of all weapons / armor / items from every vendor, if you haven't seen that item before.
-Finish Wheel of Harma challenges in Angri La for their unique rewards.
-Open all chests hidden behind doors for the Magic Key or Ultimate Key (found early in the endgame).
-Complete side-quests, with emphasis on ones that award recipe books.