First off, both comments are not entirely accurate; most laptops are only equipped with 2 slots and therefore shipped with 1 module, running in single-channel mode.
to be technical, it depends on the motherboard chipset used and the memory amount, speed, timings, and number of ranks (1R vs 2R per module) it supports.
This is always the case with any computer, but since laptops often get cheaper, stripped-down or lower-powered chipsets compared to their desktop counterparts, they're way more nitpicky as far as RAM is concerned. A wrong choice can lead to a non-booting or unstable system, and might void your warranty on the laptop or, at least, the ram.
I advise you to search for a QVL or Qualified Vendor List, to be sure you're getting...