No there are no guidelines. Because it is not only dependent on the number of concurrent users but also what they do in the model and how big your model is. I would start with small because you can easily upgrade if during the first usage you notice that the environment in to small. Just watching the metrics for a couple of days will already give you the info about the usage of your app. And upgrading is always easier then downgrading. And those upgrades can be done fast by Mendix.