However, you could use these options depending on the context:
* A stack of napkins
* A pile of napkins
* A set of napkins
* A supply of napkins
* A collection of napkins
You could also get creative and use a metaphor:
* A cloud of napkins (if they're scattered)
* A fortress of napkins (if they're stacked high)
Ultimately, the best collective noun depends on the situation and what you want to convey.