General synonyms:
* Selection
* Variety
* Range
* Collection
* Array
* Miscellany
* Medley
* Mixture
Emphasis on diversity:
* Diversification
* Spectrum
* Panorama
* Kaleidoscope
Emphasis on quantity:
* Abundance
* Plethora
* Profusion
Emphasis on organization:
* Grouping
* Categorization
* Classification
Emphasis on specific purpose:
* Inventory
* Stock
* Supply
The best synonym for "assortment" depends on the specific context in which you are using it. Consider what aspect of "assortment" you want to emphasize and choose the synonym that best reflects that aspect.