Can I use fewer than the recommended number of fasteners for exterior wall insulation?
Your material might warp could be one of the reasons they recommend to do this - I would suggest to follow their recommendation. 

While any one can ignore a manufacturers recommendation given the manufacturer has studied their product and engineered out issues with their product and tested it - I mean what could they possibly know about their own product.. right? 

That said here is my recommendation:

Cheaper to do it right the first time than it is to do it right the second time.