combining several similar queries

If I follow you correctly, you can use two levels of aggregation. The following query puts each bucket in a separate row:

select
    case 
        when cnt < 10 then '< 10'
        when cnt < 20 then '10-20'
        else '> 20'
    end as bucket,
    count(*) cnt
from (
    select count(p.owneruserid) cnt
    from users u
    left join posts p on p.owneruserid = u.id
    group by u.id
) t
group by case 
    when cnt < 10 then '< 10'
    when cnt < 20 then '10-20'
    else '> 20'
end 

Or you can get all three counts on the same row as follows:

select
    sum(case when cnt < 10 then 1 else 0 end) as cnt_less_than_10,
    sum(case when cnt >= 10 and cnt < 20 then 1 else 0 end) as cnt_10_to_20,
    sum(case when cnt > 20 then 1 else 0 end) as cnt_more_than_20
from (
    select count(*) cnt
    from users u
    inner join posts p on p.owneruserid = u.id
    group by u.id
) t