I have created a single configuration project with "This build is parameterized"
option.
o VARIABLE NAME : FOO
o goto the shell script and echo $FOO;sleep 120

Now saved the configuration and started the build using following URL...

http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST1

Now build #1 started.

While the build #1 is going on, I have started few more builds.
http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST2
http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST3
http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST4
http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST5
http://localhost:8080/job/buildWithParameters/buildWithParameters?FOO=TEST6

When the build #1 is completed, build #2 & build #3 are started together.

When the build #2 & #3 completed, build #4 & #5 are getting started together.