Jamma specifications/standards are on the AMOA website:
www.amoa.com/policies/industry/There's no rule that says the minimum current the supply should put out, but speaking as an expert, I'd want a minimum rating of 11 amps from the power supply's "+5 volt" line. Jamma games do not use a "-12 volt". They do however use "+12 volts for sound (on board amplifier chips), 12 volt coin counting meters, and sometimes 12 volt coin door lights. You need at least 2 amps (preferably 3) on the "+12 volt" line.
Many Jamma games tell you in their manuals what current ratings the power supply should have for the particular game board. If you connect a power supply too small for the job it may work for a few minutes then crap out from being overloaded. You need headroom on the power ratings.