Having a usage column would be really nice; that’d make a great feature request.
If you select an item in the project panel and look at its thumbnail, you’ll see its usage count next to the thumbnail:

This is a bit of hack, but we could use a script to stuff the usage count into the comment column. (Please note this wouldn’t update live; you’d have to run the script every time you wanted to get accurate use counts.)
// this script steps through the entire project, and puts each item's usage count in its comments column in the project panel
// we might be overwriting some project panel comments, so let's make sure we can undo this action
app.beginUndoGroup("Put usage in comment field");
// how many project items have usage counts?
var countedItems = 0;
// step through all items in the project
for (var i = 1; i <= app.project.numItems; i++) {
// check to see if they're footage items or comp items
if (app.project.item(i) instanceof FootageItem || app.project.item(i) instanceof CompItem) {
// get the usage count for the item
var usedInCount = app.project.item(i).usedIn.length;
// if it's used once or more, update the comment and update the item usage counter
if (usedInCount >= 1) {
app.project.item(i).comment = "Used in " + usedInCount + " comp" + (usedInCount == 1 ? "" : "s") + ".";
countedItems++;
}
}
}
// write a note to the info panel about how many items we've tweaked comments on
writeLn("Updated comments for " + countedItems + " item" + (countedItems > 1 ? "s" : "") + ".");
app.endUndoGroup();