For 8th notes you'd play two notes per click, 16th notes would be 4 notes per click - obviously, in 4/4 time.
When in 4/4 time (Four Beats Per Bar), You play two notes per click for eigth notes, and 4 clicks per beat for 16ths... count them like....

quarter notes... 1 2 3 4 (+ = and)
eight notes... 1 + 2 + 3 + 4
sixteenths.... 1e+a 2 e+a 3 e + a4

lol all even though, not like how i typed it.
Please help me by reviewing my lyrics
Dave_Gray is correct, but so you don't just take what he said and learn it by heart without understanding...

Here's the simple way of reasoning: the name 8th note comes from the fact that there's 8 8th notes in a bar in 4/4. So per beat (4/4 has 4 beats, logically), that's two notes. Same for 16th notes, 16 16th's in a bar, 4 per beat. You can apply this to any time signature and any note value.
He who joyfully marches to music in rank and file has already earned my contempt.
He has been given a large brain by mistake, since for him the spinal cord would fully suffice.

Remember: A prudent question is one half of wisdom.

it depends on your metronome settings...first, find out your desired speed, so that one click of the metronome is one beat (1, 2, 3, 4) of the bar. That are the 4th notes. One note per click/ per beat. To play 8th notes, you have to play 2 notes per click.
When I started practising with the metronome, I found it easier to double the metronome speed for playing 8th's and (if possible - depends on your metronome) double the metronoms speed again to play 16th notes. It works fine with almost every metronome if your 4/4 speed is max. about 50 bpm. a lot of metronomes can*t do more than 220 bpm.
so ..even when you're playing 16th notes...you can still play one note per click. Makes it a little bit easier in the beginning.