0 TDS.
The goal is always 0 TDS but you will not reach that with RO alone, that is why RO/DI systems are so popular.
RO alone when functioning properly should get you a 96-98% reduction in your tap water TDS readings. If your tap is close to the national average of 250 TDS then you can expect most RO systems to get you somewhere around 5-10 TDS, lower is always better.
RO by itself is not particulary effective at removing certain substances such as forms of ammonia(nitrites, nitrates) and silicates among other. On the other hand DI by itself is not as efficient at some things either so they compliment each other and work well as a system along with sediment and carbon filters which are required to remove particulates and chlorine.