If this surface is uneven, the pavers on top will be, too. Ideally, the sand should be 1 in. thick, but if it's a tad thicker or thinner in spots, that's okay. What you want is a firm, flat surface for laying pavers.
Sand also locks the pavers in place.
When you vibrate the pavers, they'll bed themselves slightly into the sand. If your patio is under 10 ft. wide, use a screed board with a 2-in. On larger expanses, level long lengths of iron pipe it the sand 2 in. Whichever screeding method you use, roughly dump and level the sand over the compacted subbase, then fill in low spaces and rake away excess sand as you drag your 2x4. Shuffle the screed lightly from side to side as your work.