Common and descriptive:
* Pile of seashells
* Collection of seashells
* Assortment of seashells
* Heap of seashells
More poetic or evocative:
* Drift of seashells
* Scattering of seashells
* Treasure of seashells
Ultimately, the best collective noun will depend on the context and the specific type of seashells you're referring to.