It becomes one hundred realms because the ten realms share the other nine realms (Jikkaigogu, Mutual Containment of the Ten Realms), and since there are Junyoze (Buddhism Ten Factors of Life) in each of the hundred realms, it becomes senyoze (a thousand factors).
十界が互いに他の九界を具足しあっている（十界互具）ので百界、その百界にそれぞれ十如是があるから千如是となる。