Yes..and no. These days, modern cards are very powerful, even the low-end ones. Unless you are trying to acheive triple 4K gaming and maintain 60 FPS, then you would need SLI or Crossfire. But if you have one monitor, a GTX 1070 or 1080 or R9 Fury should be able to handle 4K Gaming. If you are sticking with 1440p or 1080p, then a single card would be jsut fine, and no need to get two.
Not to mention, a lot of games are not optimize for SLI, so it's mostly for aesthetics.