The top-answerers-on-tags and top-askers-on-tags documentation claim:

Returns the top 30 answerers active in a single tag, of either all-time or the last 30 days.

and

Returns the top 30 askers active in a single tag, of either all-time or the last 30 days.

Both return 20 results. This is consistent with the per-tag top users pages, which also show just 20 users, for the past 30 days.

This is simple documentation bug; it should simply read top 20 instead.