Naps! 😴

Sarah • Proud mum to 2
Hi Mamas! I have a 20 month old son and I feel like he doesn't sleep enough! 
He has one 1hr ish nap a day. He goes down for the night between 6-630 and is up at 5. Occasionally he will be up once or twice throughout the night. 
Some days he will only have a half hour nap. I try so hard to get him to sleep longer or to have a second nap and all he does is sit in his crib and either cry or talk to himself. Sometimes he's down there for over an hour. 
How many naps do your LO's have? Do you think my son needs to sleep more? Any advice? TIA