Each difficulty is associated with a target hash, which is a 256 bit number.
For example, for diff 1.0 the hexadecimal target hash is:
0x00000000ffff000000000000000000000000000000000000000000000000000 which has 32 binary leading zeros.
As long as difficulty increases, the target hash decreases according to the formula:
current target = first block target / difficulty = 0x00000000ffff000000000000000000000000000000000000000000000000000 / difficulty
Finding the appropriate method to perform such division you can calculate the target for any desired difficulty and know the number of leading zeros.
Regarding mining pools, as the previous answer said, it depends on the miner, the hash rate, the pool... what difficulty you can or you are going to be using. Slow miners will likely use lower difficulties, so according the previous formula their required target hash is not too small for their hardware to still be capable of finding hashes that are less or equal than the target. Fast miners can calculate more hashes/sec, so they will mine at higher difficulties with smaller target hashes.