The number is the pair count and the P stands for pair. A 12P cable carries twelve twisted pairs, so twenty-four conductors in total. You'll also see T for triad, as in 2T, which means two sets of three twisted cores, normally used for three-wire RTDs.
Pairs are twisted rather than laid up loose for a reason. A pair running past a noisy power cable picks up induced voltage on both conductors roughly equally, and because the twist keeps swapping their positions, most of that noise cancels out at the receiving instrument.
Order by signal count, not conductor count. Each 4-20 mA loop needs one pair. Our instrumentation cable selection guide covers pair count alongside the screening and armour choices, and the screened instrumentation cable range shows what's available.