so things are stored on the key itself?
Yes, in a way that one-time password secrets and private keys can't be
exported out of the device for security reasons.
How many passwords can it hold, and can it do multiple for a single site?
We recommend to use one-time passwords which are more secure than
ordinary static passwords. However, for legacy reasons you can store up
to 16 static passwords on the Nitrokey. Of course you are free to store
multiple passwords for a single site.
I had a pass manager get compromised in the past, which sucks. I spent HOURS resetting almost 800 passwords because of it. A hardware alternative would be a HUGE increase in security I think.
Said for you, we aim for high security solutions instead of these questionable
web services...