how do I set the number of trinkets I give out as a thank you for dances?

It's random. Sometimes people who dance will get or

Ah, thank you. I was afraid I was doing something wrong, and not giving out enough.

You're not really giving out trinkets, so the amount is within a random allowance. :P If you notice, the trinkets in your inventory never go down, only up.

More specifically, trinkets are created out of thin air. While they are described as coming from you or from your dance partner, in reality Subeta is generating them and handing them out.