You can add some thickness to your studio production by creating a double effect that mimics the sound of the stacked instrument or vocal part. There are techniques that you can try to give you the similar feel of having the performer perform that part many times.
Short delays: a delay can give the impression of multiple takes. Start by trying a delay of 25ms where you have no feedback. Discerning a delay is hard for the human ear when it is shorter than 25ms. If your delay is longer, though, it will sound like an echo. Try playing around with delaying the time and see if you like it. If you have a modulation adjustment in the delay, try using a slow rate setting or speed to add a subtle pitch variance to the echo. When you have a pitching change, it helps to make the effect authentic. This trick to get a double guitar track is an old one.
Editing: an audio engineer student may find this one hard and time-consuming, but this is preferred on vocals more than a short delay. Make two copies of your vocal track. Go through your first copy and chop up the vocal randomly every few words. Now, go to your second copy and chop up vocals again, making sure you don’t chop in the same places as the first copy. Go back through your chopped tracks and nudge each region randomly about 20ms. Some need to be nudged early in some late. When they are random, it helps the effect. Send these tracks to the slow-moving chorus effect to add modulation. To find the sound you are looking work, you can now play around with the balances.
Plug-ins: each year, new plug-ins come out that sound better. Some of these plugins allow you to get the double effect that would normally take a lot of pitch and delay shifting. Sometimes, you can get a cool effect by using the editing and delay trick with a double plug-in.
Hopefully, these tips will help you to add thickness to your studio project.