It's very subjective, there's no numerical answer to it.
The NHS qualify it by how much it interferes with your life. If those 18 hot flushes each resulted in a change of clothes then it's a lot. If none of them had you drenched then they might just be annoying rather than demanding action.
I had night sweats because of taking fluoxetine even in my thirties, my menopause hot flushes started simultaneously with my final period age 51. Of course I didn't know then it was my final period, the previous three had been like clockwork, over the next 12 months the hot flushes became sweatier until I needed a change of clothes at every one, then it became a morning only thing and they really were starting to ease off a bit by the time I was eligible for HRT a year later.
I said I wanted HRT for my bone health as I've many risk factors for oesteoporosis, she said it's not allowed unless you're getting symptoms. I confirmed I was still having symptoms and after jumping through another few hoops I was allowed to try one type of HRT and when that made me feel worse my GP said that was the best one, all the others would make me feel even worse and I had to wait another year for the menopause clinic to phone and overrule her!
PS You ask about hot weather. Yes, I found there were many external triggers. Anything that brings heat to the body has the potential to set off the demons, even the touch of my husband next to me in bed would burn too hot, he can feel impossibly hot for a human, my perception of heat can be that far off the mark and the touch of him would send my body into the full alert of being burned, with the overreaction of instant sweating and accompanying dizziness.