[DarkDeviL]

It depends on various factors such as amount of player slots you require, what plugins you're running, and so.

There is absolutely no ways for anyone to tell this for sure; the only way is by testing it yourself, and reducing amount of servers if things starts to be lagging, showing signs of overload, etc.

It's a very old CPU, but 6 core, so ... a good guess would be 6 servers, with the typical "one per core".

However, the CPU frequency is pretty low, so taken everything into account so far, I'd probably stick to 6 x 10 slots serves at max if I were you.

If there are really 2 physical CPU's in, as your text seem to indicate, you might try 10 x 10 slots servers, but I wouldn't push it too hard as it is a very old CPU with a low frequency.

Always quality over quantity.