If I'm not mistaken, "tens of" means 10 to 99 and "hundreds of" means 100
to 999. Is this correct? I found in some dictionaries that "tens of" is
actually not correct.
An unqualified "tens" covers the range from "one ten" through "a few tens" and on to "many tens", so you can answer it by asking whether many stops at 9.9, which it doesn't. There could quite reasonably be 17 tens, for example.
I also found that "hundreds of" could also mean any arbitrary large
number. So how would people usually interpret "hundreds of"? Based on
Yes, and you might have to conclude that you haven't been given enough information to judge.
In my case, I want to describe numbers of some items that are usually
50-90 but sometimes could be around 100-200, but definitely not as
many as 300 or so. I want to really emphasize that there are a lot of
these items. In this case, can I say "... have tens or even hundreds
of ..."? Would people misinterpret what I actually want to say?
If you use "tens or even hundreds" then the "even" says that hundreds is not the usual case and "tens or even" suggests that it's the low hundreds. However "tens" can be fewer than fifty and "hundreds" could be taken as three hundred or more.
As you definitely do not wish to convey the possibility of as
many as three hundred, you'd need to be definite in saying it. The way you put it is fine - "usually 50-90 but sometimes 100-200" - or anything similar - "50-200 but usually less than 100" or "fifty to a hundred, maybe a couple of hundred".