Creating an index to rank a list of companies based on how many areas they invest in

I have a list of companies and 10 possible areas of investment for each of the companies. I would like to create an index to rank the companies based on the areas they invest in. More areas of investment would mean a higher index number. What would be a meaningful way to go about this. I thought about getting a percentage for all companies meaning number of investments divided by 10. For example if a company invested in 5 areas, it would be 5/10 = 0.5. I would do this for all companies, then calculate their percent rank and use that as an index. Does this sound reasonable?

Thanks in advance