There are both external and internal motivations that drive a person to invest in any kind of startup. The external motivations will come from things like product/market fit or doing surveys.
You are asking about the stage before you get to that, so therefore you are asking about the internal motivations that lead you to pick which ideas you want to move ahead with.
One approach is to identify which of your ~20 ideas have all 3 of the following attributes:
1. What do you really deeply care about? What are you really deeply passionate about? What could you spend all day thinking about?
2. What can you be the best in the world at? What competencies, resources and background do you have that points you towards being the best of the best in a particular area?
3. What are the best sources of revenue available to you? What are the most lucrative products, customers and markets that you have access to?
If you're being brutally honest you will find that most of the ~20 ideas will have only 2 of these attributes.
The few ideas that have all 3 attributes - those are the ones to advance further to product/market fit etc - the external motivation stuff.
This is based on an idea discussed in the book Good to Great written by Jim Collins. The book identified this basic approach as being the driving force behind the breakthrough success of a wide range of companies they studied.
The whole thing sounds obvious, but it is very difficult being honest when assessing yourself - having the assessment divided into 3 clear sets makes it easier.