How common is mental illness?

How common is mental illness in the population? I've managed several friends' houses they rent out to tenants / lodgers over the years & so I'm in the unique position to have come across a huge amount of people in a short space of time- and to get to know them very well.

One of the things that surprised me was how many of these lodgers were suffering from serious mentally illness after moving in. You'd do interviews get the best lodger each time, but shortly after moving in it would emerge that the best ones "hid" their true selves during the interview process, and showed a "fake" version of themselves.

Here's a random cross section of the first 11 lodgers, in terms of their behaviour:

Lodger 1: - Great in most ways. Had friends and was popular, but constantly moaning about her life.
Lodger 2: - Very friendly & cheerful in front of others, had friends in another country, but would hide in her room & cry 99% of the time. Would require significant notice to go out with anyone, to psych herself up.
Lodger 3: - Serious behavioural & psychological issues. After moving in, she switched behaviour immediately & it emerged she had a problem with being alone. She was quite forceful, extremely high energy, would constantly harass anyone who was in to the point if anyone spent 20 minutes with her, they'd need 2 hours sleep to recover. She'd knock on everyone's doors, ask to use up their drinks or food, jump up & down in an over-active hyper manner, walk into peoples' rooms while they were asleep, and generally behave in an extremely inappropriate manner. After a week of this behaviour, she would disappear in her room for a long period & appear extremely depressed. She would have to take days off work with "illness" whenever this episode happens. Then the cycle repeats. She had extremely dark under eye circles, refused to drink tap water & only drank sugary caffeinated drinks. No one could persuade her to change her behaviour. I felt sorry for her and did my best to help her but at the end of the day my job was busy enough and I just couldn't afford to be her therapist on top- it was so impossible to live with her that she had to be asked to leave.
Lodger 4: - Extremely depressed alcoholic with serious anti-social behavioural issues. No friends.
Lodger 5: - Low energy & slightly depressive, no behavioural issues. No friends.
Lodger 6: - Inconsiderate, immature & selfish. No friends.
Lodger 7: - Inconsiderate, immature & spoilt. Would never perform any household chores & expected people to clean up after her. If no one did it for her, she'd get stressed & confused. Lots of friends.
Lodger 8: - Possibly a high functioning psychopath. She would expect everyone to do everything for her, and never return any favour. She would eat a meal you cooked for her without saying thanks or doing any washing up. Constantly asking for huge unreasonable favours. After doing so much for her, when you finally say "no" to her requests, she would reply "excuse me??" and be quite shocked. If anyone told her a sad or happy story, she showed no reaction on her face- just a blank stare. She was incapable of empathy, and had no friends. She tricked her new bf into believing she has friends by harassing all of the housemates to meet her in a bar. Most didn't turn up due to their own mental issues, but the ones who did quickly realised she was only using them to show her bf she had friends. He had his friends there, and she had no one. After proving to this bf she had "friends", she never spoke to them again. She had no friends.
Lodger 9: - Had no friends and was shy to the point of extreme. If you speak to her, she would clam up & be afraid to be herself or say anything, even after seeing her every day for months. It didn't matter how relaxed or welcoming you were toward her- she simply wanted to be left alone. This continued for 6 months until she moved out abruptly with less than a day's notice & asked for all of her rent to be refunded. Her reason was that she doesn't like "to stay in the same place too long". She was surprised that her rent couldn't be refunded with a day's notice & that it would take time to find a replacement.
Lodger 10: - Had serious anger & anti-social issues, was overweight & would jump from the top floor down a whole flight of stairs to the bottom floor every day with a *crash*, eventually causing the ground floor of a very well maintained 1900s terraced house to give way & cave in, causing over £7,000 of structural damage. He would push past people without saying "hi", and just look at the floor constantly. He had no friends.
Lodger 11: - Very friendly during interviews, but after moving in it emerged he has no friends, doesn't smile, is very low energy & depressed, and no amount of being happy or welcoming around him would affect his mood- he'd just absorb it and bring you down.

From my first hand experience:

3, 4, 8, 10, 11 were definitely extremely depressed & possibly had other serious mental issues.

1 & 2 were relatively healthy, but just in a temporary transition in their lives that caused depression.

5, 6, 10 had serious behavioural issues.

I could go on but I won't. They're all different but almost all seem to come with different but serious mental or behavioural issues.

I'm wonder how common place serious mental or behavioural problems are among the human race? I certainly didn't realise it was that high. Going by my experiences meeting these people, it appears to be around 80%. It's quite unhealthy to be surrounded by, and I'm convinced they make each other worse & can't get along. I have to be very careful not to turn into an unpaid life coach / therapist, and have to leave to get a break quite regularly.