Ferrets don't need other ferrets to be happy, but if you won't be
around much, two or more will keep each other company.  They'll also
be more fun, but more responsibility.  Many people have three, five or
more ferrets, which may be more fun than you can take. :-)

I'd recommend getting one at first, so you can get to know it, and it
you.  There's some advantage to only having to train one at a time,
too.  I'd suggest at least a month between them, if you're going
to get several, although it's certainly not necessary.  If you decide
you want more later, you can always get another; they usually get along
just fine [4.6].  There's no problem mixing (neutered) ferrets of
either gender in any combination.