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.
If you want a job that no need to invest money in here. Just go with airdrop it is easy to earn but not good enough or not big enough. I think joining bounty signature campaign is the best for you if you want no money involved to be invested.