Due to safety concerns, IATA issues guidelines concerning the transport of lithium batteries, which airlines will likely enforce. (In a worst case scenario, a lithium battery could spontaneously explode and cause a fire in the cargo hold, which is inaccessible to humans during the flight.)
The Lithium Battery Guidance Document for 2014 states (emphasis added):
Passenger Provisions [p. 12]
Transport within Passenger Baggage
… certain replacement batteries which are not OEM or aftermarket batteries but simply low-cost copies of those – also called “fakes” – may not have undergone the required tests. Untested batteries are consequently excluded from air transport.
188.8.131.52 Portable Electronic Devices (including medical devices) containing Batteries
184.108.40.206.1 Portable electronic devices (including medical devices) (such as watches, calculating machines, cameras, cellular phones, lap-top computers, camcorders, etc.) containing batteries when carried by passengers or crew for personal use, which should be carried in carry-on baggage. Spare batteries must be individually protected to prevent short circuits by placement in the original retail packaging or by otherwise insulating terminals, e.g. by taping over exposed terminals or placing each battery in a separate plastic bag or protective pouch, and carried in carry-on baggage only. In addition, lithium batteries are subject to the following conditions:
(a) each installed or spare battery must not exceed:
1. for lithium metal or lithium alloy batteries, a lithium content of not more than 2 g; or
2. for lithium ion batteries, a watt-hour rating of not more than 100 Wh.
220.127.116.11 Lithium ion batteries exceeding a watt-hour rating of 100 Wh but not exceeding 160 Wh may be carried as spare batteries in carry-on baggage, or in equipment in either checked or carry-on baggage. Batteries must be of a type that meets the requirements of the UN Manual of Tests and Criteria, Part III, subsection 38.3. No more than two individually protected spare batteries per person may be carried.
Although the text provided above does not impose a limit on the number of lithium metal and lithium ion batteries that fall under the 2 g or 100 Wh limitation (See 18.104.22.168) being carried as spares within a passenger’s carry-on baggage it must be emphasized that the number of spares must be “reasonable” in the context of the equipment used by the passenger and his or her itinerary. Furthermore, these must be intended to power portable electronic devices (including, but not limited to, cameras and professional film equipment, laptop computers, MP3 players, cell phones, Personal Digital Assistants (PDA’s), pocket calculators etc).
Batteries which are carried for the purpose of resale or beyond personal needs are clearly not covered.
For reference, the built-in battery on a 15-inch MacBook Pro has a 95 Watt-hour capacity.
In summary, the number of battery-powered laptops you can take on a plane would be limited by what you can fit in your carry-on, and it has to be reasonable and for personal use. Check your airline's policies, which may differ from the guidelines above.