It depends. Most definitions of "genius" would be pluralized as "geniuses".

However, these alternate (and defunct) definitions would be pluralized as "genii":

1. A person who has great influence over others.

2. A "guardian spirit" in Roman Mythology that presides over a person or place.

3. An alternate spelling of "genie", the anglicanized version of "djinn", which is an evil/chaotic spirit in Arabic mythology.

As "genius" is almost never used for these contexts (because either the context itself is never brought up, as with 1 and 2, or because the alternative spelling is far more popular, as in 3), it's safe to always pluralize "genius" as "geniuses".

There are too many geniuses in this room!