Detect it only 1 channel about jack-matchmaker

[SpotlightKid]

If there is only one channel, can jack connect this one channel to the all(LR) channels of the system playback port?

Yes, it can.

You can either specify ports explicitly via the -e option:

jack-matchmaker -e \
    "My Application:out_1" system:playback_1 \
    "My Application:out_1" system:playback_2

Or you can use a regular expression for the output ports to connect to:

jack-matchmaker 'My Application:out_1$' 'system:playback_[12]$'

Notice that the -e option is not used here, so all port specifications are regular expressions. Also note the $ at the end of each regular expression, so that e.g. the second one matches system:playback_1 and system:playback_2, but not system:playback_11 or system:playback:24.

For the syntax of Python regular expressions, see https://docs.python.org/3/library/re.html#regular-expression-syntax.