segmentation for model

[barakmiz]

i am buiding a model for company with large number of customers (milions),
i am facing the need to segment the customers and then build model for each segment.

1. how should i make the segmentation for the model ?
2. how can i compare betweeen several segmentations options which is better ? what are my kpi to compare between them ?
3. if i decieded to segment the population by two paramters how can know which one will be first.
4. how many segments should i build ?

comment : the segmentation is in the context of the specific model and only for it , and not just general segmentation of the customers.