What type of field are the numbers in? If they are in a calculated field, such as a formula field, lookup, or rollup, there might be other options.
If the field is a text field, here are some more formulas for splitting a comma separated list into separate fields. If you go that route, you would need to use the VALUE function to turn the string into an actual number, and have another function to calculate the quantity of numbers and the average. It’s not for the faint of heart, but it is doable if the maximum number of items isn’t too high.
A script could do the calculations much easier. However, it would also require someone to press the “run” button.