i) Each cell transmit one of 168 unique sequences on its secondary sync channel (each bit sequence is 62 bit sequence)
ii) The Primary Sync channel carries one of three unique sequence (62 bit Zadd-off chu sequence)
iii) So total number of Sync code combination become 168 x 3 = 504 and each of these combination become a physical cell ID of a cell.
For the location of Sync signals in downlink frame structure, see Downlink Frame Structure page.
If you are interested in how these synchronization signal is physically generated and how it looks like in constellation, refer to following pages.