If you want buy btc now , btc price already at $690, except you bought it when btc price still was at $500, you can proited more. But alot people believe if btc price would be going higher (maybe $1000) on the next year, so buy and hold your btc now. 
Long term invest is really a good type of investment and 1 bitcoin is already enough is can't able to buy more.
But for starting, that is enough. And most of the bitcoin rich people now, long term investment is one of the ways , on how did they became a millionaire.
If you will go into long term investment, you can't feel more the profit if you only have BTC1 on hand. As a start it's fine but remember to add some for it while you are on a purpose of long term investment. Those rich person you mentioned are not holding only BTC1 for long term investment. They are in the possesion of lots of coins so whenever there's a price rally they can really feel the profit.
Yeah for starting 1 btc is enough but in the long run, you will realize that you need to put more capital in able to get bigger results for the harvesting time. Yep those rich bitcoin people are hold thousands of bitcoins in their wallets. And it is very inspiring on how they did become millionaires, if they did we can also.
