Hey there,
It is really up to you. Usually, MetaTrader will allow you to put no more than 99EAs in one account. So you will need multiple MetaTrader platforms on your computer if you want to test the EAs from every new month.
We had a topic in the forum about how you can install many MetaTrader platforms:
Now, I would suggest you not to make it too complicated.
Keep just two MetaTrader accounts (one simulating real trading). And test the new EAs that come every new month. Place in the second Demo, the EAs that perform the best at the moment. Fifty is a good number of EAs to test. It is enough :)