Is there a way to create a report on how many users posted new queries each month?

Oh. Yeah. That’s not what my (SQL) query does. Guess I misunderstood. You can see how many topics are posted each month by going to /admin/reports/topics and selecting the chart mode. You can also see engaged users on /admin/reports/daily_engaged_users, but that counts replies and likes. If I understand correctly, you are looking for something like this:

select to_char(created_at, 'YYYY-MM') as month,
       count(distinct user_id) users,
       count(distinct id) topics
from topics
group by to_char(created_at, 'YYYY-MM')
order by to_char(created_at, 'YYYY-MM')

That does require the Data Explorer, which might require a sysadmin installing it.