In total, you can hold up to 8 items. At most, 4 of those items can be in your ready slots. You can have more than 4 items in your pack however, if you have fewer than 4 items readied.
For example, if you see that a combat is going to have a lot of enemies that will affect your ready items, you may choose to put some of your more valued items in your pack, and go into a battle with 2 readied and 6 in your pack. Or even, all 8 of your items in your pack.
