as many as possible, if you have just two or three, the smallest one will likely to get constantly bullied and may wind up dead.
Exactly what happened when I had just three, the smallest one became so skinny and died.
The second biggest one was skinny as well, so I was quite worried, got another three small ones, so total of five.
Then instant improvement, they are always out together and the skinny one started to put on weight, which was really good to see.
Now the biggest one is always the loner, but never gets bullied.