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.
Good point. It's not actually the figure that matters, but the amount you can shed off as capital that you are open to all scenarios. If it prospers and amounts to a profit, then well and good. However, if it would make the wrong turn, and you would lose your profit, it should not be an amount that you would actually weep and cry over and you would despise investing. You must take note that in any venture in the cryptocurrency market, any move may amount to a win or a loss, like any other business venture.