How much smartphone should you use in a day?

No maximum, no minimum. Smartphone displays are purposely designed to be as addicting as possible (more time = see more ads = buy more product). Relationship between cell phone time and brain tumor, psych disorders debunked long ago.

Trying to keep it limited at the least amount possible daily would allow for daily activities meaningful time. There are definitely behavioral effects that we see in children and teenagers, but affect all of us involving attention span and immediate memory.