In some (but not all) situations there are gains to be made in performance by limiting the total number of different sprites objects in your game, but there are also other good reasons for containing multiple objects on a single sprite, mainly by making your project a little more readable and simplifying the code requirements.
There are a few factors to consider before determining what the best solution would be:
1) I would tend to only include objects on the same sprite that appear on the same layout as you would otherwise be filling up your memory for each layout with unnecessary art assets.
2) I would only include objects on the same sprite that have simple behaviours and use the same basic logic e.g. all inventory items, otherwise your code will become overly complex trying to differentiate between the objects when you could instead just refer directly to a specific sprite.
3) If there were groups of objects that appeared on different layouts with similar behaviours, or objects with quite complex behaviours and multiple animations on the same layout, then I would prefer to put them on separate sprites and add them to a family so that I can still use the same code groups to control them.
4) In certain circumstances I would use the individual frames on a single animation for different objects e.g. scenery objects, non-animated pick-ups. This also makes it quicker to import graphics as you can import a sprite strip to a single animation instead of individual frames to separate animations.
5) It is much easier to perform actions like spawning random objects when they are on a single sprite: for example, if I had 10 pickups and wanted to spawn a random one then if the pickups were on separate sprite objects I would need to assign a random number to a variable to determine the pickup and then have a string of 10 conditions to spawn the correct one; if instead I had each pickup on a single frame or animation then I just spawn the sprite and pick a random frame or animation - 2 actions on 1 event as opposed to 10 conditions.