This is if the diploid number is 18.
In humans, the number of chromosomally different zygotes possible are (2^23)^2 (2 to the power of 23, and this value squared) which equals 70,368,744,000,000 different combinations without taking crossing over into account. 23 is the haploid number for humans, and therefore for an organism with diploid number 18, the value would be (2^9)^2 which equals 262,144 different combinations possible. If crossing over occurs once, this value becomes (4^9)^2, which equals 68,719,476,740 different combinations. But you should also remember that crossing-over can occur several times in each chromosome.