Growing Mint

I have many mint plants plus the ones I have just ordered. How far apart should they bee from each other so that they will not cross?

If you are using the mint plants for your own use, making teas and using in cooking, there is no need to be concerned about the mints cross breeding and then using the seeds the following year.
The mints spread quite freely and if you are still concerned about them growing into each other, I would suggest to plant each variety in large individual pots. This is what I do at home, where I have three large pots in the shade.