Here are some options, depending on how you want to describe them:
Descriptive:
* A bed of chillies
* A patch of chillies
* A basket of chillies
* A string of chillies (if they're strung together for drying)
Figurative:
* A fiery bunch of chillies
* A blazing collection of chillies
* A searing heap of chillies
Ultimately, the best collective noun depends on the context and what you want to emphasize.