Question: I have lurked for a time on security websites and similar. I came to wonder, with the number of passwords we have (or websites that need a password) how to manage this.

Since I have well over 400 sites where a password is used, how can I keep track of all the passwords and avoid reuse?

I have a password manager but still have passwords that are used way too many times. Is the best solution to just make 400 different passwords and let the password manager deal with the issue and look them up every time I want to login? 

Or is reusing for minor accounts the best alternative in terms of usability?

Answer: 
The short answer is yes, let the password manager handle it.

You should have your password manager generate a unique password for each site. Don't make them yourself and you won't have to worry about reuse. For sites you currently reuse on, incrementally replace them as you come across them. Many have tools to identify password reuse which can also help you.

Based on your comments, you seem hesitant to commit to all random passwords because you use several different devices. Many password managers offer syncing solutions that work very well. Both Android and iOS support filling in login forms with password manager apps. If manual entry is a sticking point, consider generating [pass*phrases*](https://www.eff.org/dice), which are easier to remember momentarily.
400 sites are a lot, I do not use that much, but I just let pw manager deal with passwords.

However I have some sites that I use frequently and don't want to login everytime in the password manager. For those sites I use a simple method to create easily remember passwords.

It is actually two methods.
I create a whole sentence and the first word usually the name of the site, a funny sentence coming from the function of the site or the title of the site. A whole sentence including some numbers or signs no way to crack, except brute force.

The other method is take a poet and the first letter of every word in a verse gives you a nice hash looking-like password. you can tune it up with numbers or signs.