Determining how many accounts you can run isn't really possible, as all scripts are not equal in terms of resources required. As well, you didn't specify if you were using the Mirror Client or the original OSBot client, which would greatly affect the amount of bots you can run.
As a general rule you can run more bots on Linux than Windows as Linux can be very lightweight and take up very little resources which would increase the amount of bots you can run. There is no best distribution, however I can personally recommend Debian 7 32 Bit, with the LXDE Desktop Environment!
If you have any further questions, feel free to post here, PM me, or add my Skype