As deep as you can: More is always better...For example, kicks separate from snares, separate from hats is great if you can do it. Beyond that: Separating out individual voices of harmony parts? Each part of a double / triple? Multiple mics on a drum kit? All are reasonable, nothing goes to waste!

If your musical contribution is in the form of a big set of tracks (like a full song), then it can also be helpful to include stems (or “submixes”) in your working file along with the individual tracks: Groupings of tracks like “drums”, “guitars”, “strings” “BKG Vox”, etc. Stems can make it easier for people on smaller systems to work with your tracks, and stems will also keep your balances and effects intact for remixers who don’t intend to change them. And they make it easy for anyone to assemble a “mix minus vocal”, for example. So...Stems are great, too!