the once means you place one exertion token per rohan companion spotted. it is true that it is a bit redundant, since I'd say once is the default, if it were more, it would have to be written (twice, thrice etc.)
The confusion might arise from the article "a". It is not equivalent to "one". Even though I'm not a native speaker, I understand a as an unspecific article, which can denote any minion (in this case) so even several ones.