sed to replace only one istance in a file at a time

Hello!

Let's say I have the following text file:

"hello hello hello
hello hello

hello hello hello"

and I would like to substitute the different instances of "hello" one at a time. (this because I want to replace them with different words).

How to do it?
sed has given me a headhacke so far...

Thanks a lot!
Rama

My personal "network" this time worked faster than forums, here is the answer for anybody who may need it, thanks vicky!

num=`grep -n ciao test | head -1 | awk -F: '{ print $1}'`; sed "1,${num}s/ciao/bye/" test