The monthly visitors flowing into a website, is a very good indicator of how well it will perform with the many different advertising networks. For example, if you want to make the slightest amount of money with adsense – and similar programs – you will need just around 10000 visitors per month. This still won't earn you any serious revenue, but its a good start for any website.
How to know the monthly visitors for a website
If the website isn't owned or operated by you, it will be hard to know how many true visitors it has per month. There are websites that try to give an estimate, but the statistics they provide are generally very inadequate and unreliable.
If you have administration access to the website, you might be able to install a website plugin such as google Analytics, it gives you very adequate insight into your websites visitor count, where your visitors are coming from, their browser and operating system, and so on.
How many visitors per month should you have?
There is not really a limit to how many you should have, as more traffic translates into more ad revenue. You can however set yourself some reachable goals – start by aiming for 10k visitors per month, as its often around here things start to get interesting, then aim for 20000 visitors per month in next round, and so on.
You may however find that there is a limit, as of how many visitors the hardware hosting your server can handle. Its unlikely that you reach this limit with these small numbers – you will most likely need thousands of visitors per day to be anywhere near your hardware limits – But it also depends on the type of website you are running. Shared hosts can also perform really bad – if you are hosting on a shared host, it may even be better to just host your own website, despite your slow Internet connection.