How many hours of sleep should a 16 year old get?

As much as needed. Adolescents normally need more sleep than adults and this is not uncommon at all for them to sleep in late because of this. I believe it is estimated that adolescents need up to 10 hours of sleep a day especially while growing. I have had 3 teens at home and can personally vouch for this :-).
A lot. Adolescents need ~10 hrs of sleep on average. Some need more some need less. Unfortunately, many teens stay up late playing video games or going on social media, & skip proper sleep. This is called behavioral dyssomnia. Poor sleep hygiene is a common pervasive problem in teens.