Let's do a power budget for a laptop...
CPU and GPU - Lots of watts (10-50W) at around a volt.
DDR, chipset, WiFi, etc - A few voltage rails like 2.5V and 3.3V, I'd say 10-20W.
HDD - SATA spec requires 5V and 12V, although most 2.5" HDDs do not actually use the 12V supply, if it is not there your laptop will only work with some HDDs, so you have a marketing problem...
DVD drive - same problem, but worse, as spinning a large CD or DVD at high speeds requires about 10W...
Fans - 5V or 12V
Stuff connected to USB - People see USB ports, plug in USB devices drawing the max current as per spec, and expect it to work.
Onboard audio and loudspeaker amplifiers - 5 or 12V
Display backlight - This one is quite the power hog, for CCFL it will use an inverter, for LED backlight it will use a LED driver obviously. Input voltage should be taken from battery without further conversion, as this is the most efficient option.
OK... So you have to design a battery-powered solution for this.
If your "laptop" is a actually a smartphone or a tablet like an iPad, no HDD or DVD drives, just a few flash chips on the motherboard instead... a very efficient ARM cpu... no fans, no 12V... things are simpler, and one 3.6V Lithium cell is likely to be the simplest solution. The only voltage which needs a boost DCDC will be 5V for USB host, but the current is rather limited.
For a standard x86 laptop though, you want to use a battery voltage above your 12V rail, so all your DCDC converters are buck, which are more efficient than boost. So you're gonna have to use 4 or more Li cells in series.
If you don't have DVD drive and can spec the HDD then you don't really need a 12V rail, but you still need 5V for USB, so you could use 2-3 cells.
Your largest DCDC converter will be the CPU VRM which will output lots of amps at around 1V. You really want this one to be efficient, and buck converters tend to degrade when the Vin/Vout ratio becomes too high, also low voltage MOSFETs have higher performance... this means you don't want your battery voltage to be too high.
Everyone seems to use either 4 or 5 cells, ie 14 to 20V, roughly. It's the sweet spot for efficiency.
Then you shop for batteries, decide whether you want custom made pouch cells to make your McBook Air fashionably slim, or fat cheap 18650s...