There is no ideal number. My favorite advice is always this: Invest an amount that wouldn't hurt you financially even if you lost all of it. Although we all hope for the best and want to make profit, the crypto environment (especially the altcoin environment) is highly volatile and you never know what might happen tomorrow.
I just want to add that the best strategy is to diversify your investment. Invest in something that is established and after you make profit, that the time to invest in other crypto.