The way you want to do this is to first tell GenieACS you want 0 instances, and then specify the params for each instance. For example:
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[]", null, {path: 0});
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[BridgeEnable:true,Alias:Inst1]", {path: now}, {path: 1});
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[BridgeEnable:true,Alias:Inst2]", {path: now}, {path: 1});
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[BridgeEnable:true,Alias:Inst3]", {path: now}, {path: 1});
The params need to be unique for this to work correctly. Otherwise if the you have something like this:
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[]", null, {path: 0});
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[BridgeEnable:true,Alias:Inst1]", {path: now}, {path: 1});
declare("InternetGatewayDevice.Layer2Bridging.Bridge.[BridgeEnable:true,Alias:Inst1]", {path: now}, {path: 1});
You would end up with only 1 instance, because the parameters are all the same.