The full example:
===
import gettext

gettext.bindtextdomain("pygettext_test","./locale")
gettext.textdomain("pygettext_test")
_=gettext.gettext
n_=gettext.ngettext

n1=1
n2=3

print n_("there is %i command","there are %i commands",n1) % n1
print n_("there is %i command","there are %i commands",n2) % n2
===

the result of n_("there is %i command","there are %i commands",n2) will be:

"there is %i command"
or
"there are %i commands"
or
different string from translation file in case of other language then English as in Arabic we have 6 forms, 3 of them don't include %i.

"لا يوجد أي أمر"
"يوجد أمر واحد"
"يوجد أمران"
"يوجد %i أوامر"
"يوجد %i أمرا"
"يوجد %i أمر"

(You may not see the Arabic well aligned because it's a RTL language).

So format string is open to fit all languages.
That's why mapping good only for predefined strings.

Then these will be our solution:
===
print n_("there is {0} command","there are {0} commands",n1).format(n1)
print n_("there is {0} command","there are {0} commands",n2).format(n2)
===