I don’t think this can be changed easily with CSS, as you can see here https://github.com/discourse/discourse/blob/main/app/assets/javascripts/discourse/app/widgets/header-topic-info.js#L156 the maximum number of users shown (when there are no tags in the PM header) is 10, as you guessed.
The avatars to show are set based on this logic https://github.com/discourse/discourse/blob/main/app/assets/javascripts/discourse/app/widgets/header-topic-info.js#L169
Maybe there is something you could do, but it would require complex Javascript, or a custom plugin.