One option is to use the raw i18n output and put HTML in the i18n:
en:
people:
one: <strong>%{count}</strong> People
other: <strong>%{count}</strong> Peoples
Then in your translation use:
<%= raw t(:people, count: 4) %>
It's not ideal but your use case might be simple enough where this is best solution.
Another would be to use two different translations and nest them, one for just the number wrapping it in the strong tag, and another for the pluralization of the word, passing in the strong HTML, similar to how you do links:
text_with_anchor: This is a link. %{href} to click it!
t(:text_with_anchor, href: link_to(t(:another_translation), some_path))