I want to connect 100000 number of devices to IoT Foundation which is binded to bluemix application runtime. node.js or java liberty. and I will bind weather insight service on this application so that each devices can ask the weather information related to its geolocation.
I wonder how many instances and memories in each instances do I need?
IoTF is built on Node-RED (which is a flow editor that runs in a Node.js runtime) and Cloudant No-SQL Database. You say that you want to bind this IoTF application to the Insights for Weather service in order to retrieve weather information and send it to all the devices connected to the Node-RED flow. You are going to do this using REST APIs I guess, since I can't find a node for doing this. Moreover, you should consider what kinds of devices are you going to use and which protocol are you going to implement (mqtt, serial, etc..). At last, you talk about a Bluemix runtime that is bound to the IoTF application, you should analyze what this is supposed to do and how it is supposed to communicate with the other components. All above has to be considered to answer your question, since all those could be potential bottlenecks and they have to be analyzed to understand how to scale.
I think the best thing you can do is to start with few devices and try to identify the possible bottlenecks. Then you could try to increase the number of devices and see how the application responds, even scaling using Bluemix features (both vertically and horizontally).
The following are some useful pointers you could use to start your investigation: