A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1206576 can be evenly divided by 1 2 3 4 6 7 8 9 12 14 16 18 19 21 24 27 28 36 38 42 48 49 54 56 57 63 72 76 81 84 98 108 112 114 126 133 144 147 152 162 168 171 189 196 216 228 252 266 294 304 324 336 342 378 392 399 432 441 456 504 513 532 567 588 648 684 756 784 798 882 912 931 1,008 1,026 1,064 1,134 1,176 1,197 1,296 1,323 1,368 1,512 1,539 1,596 1,764 1,862 2,052 2,128 2,268 2,352 2,394 2,646 2,736 2,793 3,024 3,078 3,192 3,528 3,591 3,724 3,969 4,104 4,536 4,788 5,292 5,586 6,156 6,384 7,056 7,182 7,448 7,938 8,208 8,379 9,072 9,576 10,584 10,773 11,172 12,312 14,364 14,896 15,876 16,758 19,152 21,168 21,546 22,344 24,624 25,137 28,728 31,752 33,516 43,092 44,688 50,274 57,456 63,504 67,032 75,411 86,184 100,548 134,064 150,822 172,368 201,096 301,644 402,192 603,288 and 1,206,576, with no remainder.
Since 1,206,576 cannot be divided by just 1 and 1,206,576, it is not a prime number.