Well, I started trading altcoins with an ammount of 0.5 eth.  I got it when i sold my ACC token. When i have acquired my eth, i bought some Token that i believe it will soon rise its value.  I have bought it 0.2 eth and sold it for an ammount of 0.3 eth. I think there is no ideal number except for having ammount greater than the payment for the GAS including deposit and withdraw, in addition you should have enough eth to buy other coins. And if I am to suggest you should have 0.1 eth or greater to fully maximize the capability of buying tokens then solding it at a profitable ammount. Good luck mate.