The Mega should be able to control 15 servos (the UNO can only control 12). However, in addition to the Mega, I would suggest getting a Mega Sensor shield, such as http://arduino-direct.com/sunshop/index.php?l=product_detail&p=195
. This shield sits on top of the Mega, and it provides 3 pin headers for each port which matches the servo's 3 pin connector.
In addition, the shield has a power terminal to power all of the servos. Otherwise, you need to take care of doing the wiring, connecting all 15 ground wires to the common ground, connecting at 15 power wires to the servo power supply, connecting each individual signal to the appropriate mega port, and connecting the grounds of the two power supplies.
If you go with either of these multiplexors, I would verify that the Servo library actually supports it. I would suspect the Servo library can only deal with the normal pins, and can't deal with extended pins (which you probably have to access using a library they supply).
This should work. It too provides a separate power port for the servos, and presumably connects the grounds. Note you will need to assemble it, so make sure your soldering skills are up to date. I did notice that right now they don't have any in stock.
But still I don't know yet about the power issue within above component.
What I want to ask you is the reliable way to control a lot of servos in one Arduino (will be obviously less than 15 but more than 10).
See the tutorial that I quoted at the beginning. If you go with one of the two shields, you need to simply plug in your secondary power source to the two terminals with the correct polarity.