Whether or not that's enough storage space for you is dependent entirely on how much stuff you have.
To answer the other question, it all depends. Two HDDs gives you a bit more security against HDD failure, especially if you use one as your primary drive and back it up regularly to the other. At the same time, two HDDs will be twice as noisy, use twice as much power, and produce twice as much heat as just one.
It really comes down to your preference. Personally, I like having multiple mechanical drives (plus an SSD for boot) precisely because I can back my SSD and primary storage drive up to the other, so I've got an insurance policy in case of failure. Then again, I also already had the one HDD from my old computer that wasn't...