By beating the Gym Leaders in the various towns
in total 16 badges
buy them from the poke mart if you have a lot of money.
8........... like always
in every game except gold and silver and crystal there are 8 badges, gold silver and crystal have 16 cause you can fight the kanto and johto leaders.
You need 4 badges.
The underground started in diamond and pearl so there is no underground in crystal.
You buy them at the poke-mart once you have enough badges
Beat all the gym leaders.
You go challenge the Elite Four.
there are 8 badges in Pokemon ruby like all the others. :) tehehe
In poke marts
AnswerThere are two Pokemon Leagues in Pokemon Crystal. These correspond to the two regions you explore. Kanto has the Indigo League while Johto has the Johto League. Once you have collected the first eight badges of Johto, you can challenge the first Pokemon League, and after you have collected the eight badges of Kanto, you can battle in the Indigo League.