As Rocketmagnet mentioned, just because a motor is rated at 2.5 W doesn't mean it will be pulling 2.5 W all the time. Most robots have at most 1 or 2 servos that are running at full power at any one time; the rest have very low mechanical loads (and therefore pull much less electrical power) or are "off" and therefore pull practically zero electrical power.
This leads to 2 very different approaches to power supplies:
Tethered robots and desktop computers use a power supply and heat-sinks that can handle the maximum possible worst-case power draw --
when everything pulls the maximum power at the same time.
27 servos * 2.5 W @ 5V requires a 5 VDC and at least 14 A power supply
(or perhaps several 5 VDC supplies that add up to at least 14 A).
Autonomous robots and modern laptops use a power supply and heat-sinks that can handle some thermal design power.
Some human arbitrarily picks some the TDP, which is much smaller than the worst-case power, but somewhat above the power required in "typical situations".
Then the power supply is designed so it can handle any load from 0 to slightly above the TDP.
And the rest of the system is designed so it never exceeds the TDP --
except perhaps for a few milliseconds.
The simplest approach is to have something that measures the total current draw --
then when the current exceeds the TDP, assume that things have already gone horribly wrong, and shut everything down for a few seconds.
More sophisticated approaches measure the current of each motor individually:
When some motor stalls, "limp mode" kills the power to that one motor,
so the robot continues to use the other motors at full power.
When lots of motors pull a total current that is too high,
"tired mode" reduces the power to all the motors
so the robot continues to use all the motors at a slower speed.
5 V fuses?
You could install one big 14 A fuse. Or you could install 27 individual 0.5 A fuses, one in the +5V power line of each motor. Or both. You'll probably find it easier to find "12 V" or "250 V" fuses, which will work just fine in your application.
There are many cheap polyfuses available (designed to protect 5V USB ports from excessive current). Alas, polyfuses take several seconds to "blow" -- too late to protect stuff from permanent damage, but quick enough to keep stuff from heating up, catching on fire, and burning down your house.
How to do a simple overcurrent protection/circuit breaker circuit for 12V 1-2A?
convert 12 V to 5 V
Most people using servo motors use an off-the-shelf DC-DC converter to convert whatever voltage the batteries supply to the 5V required by the servos.(c)
I see that some 18650 battery box (a) include a little DC-DC converter to convert the battery power to 5 VDC "USB battery charger".
(A few people use servomotors designed to be connected directly to 12 VDC. a)
Many DC-DC converters are set up so that they never pull more than some maximum current from the battery -- when the motor connected to their output stalls, the converter switches to a "constant-current" mode at some lower output voltage, pulling less power from the batteries.
If you put such a DC-DC converter on each servo, it automatically goes into and comes out of "limp mode" appropriately.
"Selecting the proper battery for your robot" (a)
"Robot batteries" (b)
"Batteries I use in my Robotics" (c)