$10-20 is quite enough for alts if you are a starter. That will give a sense of how trading works, how you earn and lose when the coins go up and down. But if you are aiming to earn much money, you need to invest more. However, as I said, stick to a lower amount if you are starter, because until you learn all of the nuances, you may lose some money. If your initial investment is not too big, you obviously won't lose much!   But good ROI needs good investment, so keep that in mind!