Predicting crowd density using real time streaming data
Are there any incremental ML models which could be suitable?


You have to be able to access the model when it's in production mode and actually be able to update it with a new version. all ML models should be adjusted when new data comes because of the so-called data shifting. you could periodically evaluate how much the model is performing.  

I suggest modeling the problem as a sequence model, use multiple output at each day to predict densities for each postal code.