For sneaking past multiple guards you only have to make one stealth test. From page 13 of the rule book...
If the square the ninja model is moving to or from is within multiple guard models’ AOI, then only a single test is done to avoid being detected by all the guards.
The target value is the highest awareness statistic of the guards. For each additional guard model’s AOI the ninja model is in, the target of the test is raised by 1.