I would highly recommend avoiding alt-coins. And it would not spread the risk to buy multiple alts since they are all based on the same technology. Want to diversify? Buy some bitcoin, gold, real estate, stocks, etc.
Yeah, I think investing in only bitcoin is enough for making crypto investment. The altcoins are just too risky to invest for long-term and I also believe the best way to diversify is to invest on other form of investments such as real estate, stock, and gold - not the other way around.