Just think 1gb ram per bot and you will be fine =) it will be around 0.5-1 gb per bot (atleast for me) but to not make it lag I would say just think 1gb per bot
So obviously:
32gb = 32 bots
64gb = 64 bots
I ran 6 bots on 4gb ram, but they did get lag spikes so it didnt give max performance + they crashed once in a while due to my ram being at 95 % + all the time :/ got 12gb now and 6 bots is like 6gb ram + I have shitloads of other stuff running (chrome x10, skype, steam, msn, bla bla bla bla)
I hope this help you!
Edit: Obviously running all the wow clients on lowest usage.