What I find in Drake API is that 'GetPositionLowerLimits' and 'GetPositionUpperLimits' does not support get by ModelInstance as other functions.
Anybody knows how to query this when I have multiple robots and I am interested in the Lower and Upper Limits for every robot?
I believe you can take the result of GetPositionLowerLimits (or GetPositionUpperLimits) and feed it through GetPositionsFromArray to select just one model instance at a time.
The model instances documentation has some more information.