yes 90 accounts per one person is a little bit too much. Let's say these accounts are fully loaded with campaigns and your job is just to optimize them. If you work regular 8 hours a day, then you would have left 5 minutes/per day for each account to manage. You cannot do much in 5 minutes, you need maybe 30 minutes - 1 hour for one account - again in the best case scenario. Then you could watch after a little over 10 accounts a day. and you would need 9 days to look at all accounts - again, just watching accounts and minor adjustments.
If you create/edit something in one account and come back 10 days later and find there has been major increase in clicks day after your changes, it is too late.
So yes, in my opinion, that is too much.
That's a little trick discussion I'd say, since we can have people like your boss (who obviously think that's normal) and people like myself, who don't.
Personally, I think that if you don't have an automated system to look at 90 accounts, that's totally crazy for a single person (actually even with automated system, that's too much).
That's my opinion and you should consider listening to some other folks, but I'd suggest for you to talk to your boss and hire some more people to help you with.
Hope it helps.
Leandro Martinez | Basta1Click