A “manageable amount” is obviously subjective and will vary for each person based on things such as time available to dedicate to trading. Feel free to do your own research to find the right exchange for you. I tend to value user experience of an exchange over the amount of coins on it. Ultimately, what exchanges you use is going to depend on your own personal preferences. GDAX and Poloniex will provide sufficient resources needed to be a successful trader, so they are definitely a good place to start.
No coin will go up forever, even Bitcoin has very good days, and then some really rough ones. The crypto space is ever-changing and evolving, with new opportunities coming up every day. If you believe in a coin, holding it for long-term returns is a good approach, but if you are looking to make money by trading, you cannot have emotional attachments with any coin.
It is important investors realize not all exchanges and brokers that offer delivery of the underlying Bitcoin are created equal. Some firms have fallen victim to theft by hackers who have stolen Bitcoin belonging to clients whose money was held at the exchanges. Meanwhile, other Bitcoin exchanges have gone bankrupt (as in the case of Mt. Gox), as a result of fraud or mismanagement.
Never put all your eggs in one basket. Diversify. While the potential to earn more is increased with the amount of money you invest into a coin, the potential to lose more is also magnified. Another way to think about it is to look at the cryptocurrency market as a whole; if you believe that this is just the beginning, then more than likely the entire market cap of cryptocurrencies will increase. What are the chances that this market cap increase will be entirely driven by one coin vs. being driven by many coins? The best way to safely capture the overall growth of cryptocurrency is to diversify and reap the benefits of growth from multiple coins. Also, fun fact — Between January 2016 and January 2018, Corgicoin has increased by 60,000x, and Verge has increased by 13,000x. During the same period, Bitcoin has increased by 34x. While you would have gotten impressive gains from Bitcoin, expanding into other coins could have landed you potentially larger ones.