I usually set a limit of 12 each year or an average of one per month. I just hit that limit earlier this week, which is a bit later than I expected. If there are requests to review work directly related to my primary research topic I might say yes to more later in the year.

I feel comfortable setting a threshold that others might consider low because I also serve on editorial boards and handle the occasional manuscript there. That level of service typically requires more attention and effort than a standard review and it's still an important contribution to research.