[renowner]

There is no limit to youkai in reserve. I myself have 21 total. The game only allows the top 12 to be active at any time, however.
Currently what people do is legend extend and put 80 points in faith so that they can contract every youkai. . Then legend extend again to go back to their normal build, retaining their contracts.

You still retain the 12 limit regardless, which requires I believe 35 base faith to achieve.