how many active users of this site in a month? any idea?

Of the people who either write a lot of stories, ask questions, and leave replies, it feels like about 150 or so, but of the 300 people listed as being here recently, about 40% of the names I don't recognise so either they're new, in disguise, or just never post. Lots of people come and go so, over a month, it wouldn't surprise me if around 500 people post at least five things.

If you wanted to know the exact number, you could use a web whacker and then do a bit of data manipulation.

less than 300
times x that with their multiple accounts ....

3 ... All the others are bots

hundred BILLION