Can i take remeron (mirtazapine) for insomnia as need and not on daily basis?

Yes: Unless it is also being prescribed for depression.

"Can" vs "Advised"?: Sedation is a side effect of this rx, not a primary goal. Prn use is also a variation from the original. Changing your therapeutic plan with major drugs w/out consulting the doctor puts you both at some further risk in my mind. But there certainly a large number of users who write online testimonials for this practice. I imagine that a big part of an infrequent prn use effect is placebo.