I was wondering if i can use more than one servo extensions for one servo.
thanks

Yes, but I prefer to use one extension, of the correct length.

My lhs carries extensions at 6", 12",18" and 24".

You can but you open the door for more probable problems as time goes on. Any connection ( extensions creates more of them) will eventually deteriorate enough to cause problems, especially if buried where they do not get cycled (mated/demated) periodically to wipe any corrosion/dirt from the contacts. It is always better to just splice (solder) extension wires into the lead where they are buried inside structure and not repeatedly cycled.

I've done it with no problems. What everyone else is saying is worst case. Just remember if you ever develop problems to remember the hidden connections

I had problems with extensions and started to just splice in the wire needed to get the length I wanted. Only takes a few minutes and never has any problems.

If you are worried you can coat the connection surfaces with a product called CorrosionX, which is designed for just that problem (prevent contact oxidation). The military and aircraft manufacturers use it as well.

Its not the best way but its perfectly acceptable and works just fine. Just make sure to tie the connectors together or use a safety clip so the connectors can not separate over time due to vibration.

The less connetions you have the better but there is no reason you can't use more than one extension . If you know how to solder you would be better off to buy a length of servo wire and solder up one continuous line .

I was just thinking more on this and almost all of my aileron servos have three separate & connected runs. 6" to 12" to get from the receiver to the wing bay. I connect there when assembling at the field, and the line in the wing will have a jumper to get a few inches loose wire so they can be connected. Have done this many, many years. Much easier that trying to plug directly into the receiver. I mark the mating connectors with like-colored cable ties.