The add-on is subject to the quota limitations of the account that installed it.
That is different than how many users have installed your add-on.
There's a difference between individual add-on installations, and the number of users for a document.
Millions of Google accounts can install your add-on.
Theoretically, an indefinite number of users can install and use your add-on.
But that's different than the quota limits placed on the individual account.
If a particular Google account installed your add-on to their spreadsheet,
AND that spreadsheet is shared with lots of other users,
from the account that installed it, then all those users are going to use up the quota limits
for the account that installed the add-on.
The account that installed the add-on doesn't' get extra special quota limits because it's an add-on.
As far as quota limits are concerned, your add-on code runs exactly the same as if the account that
installed the add-on was actually running the code as their code.
Actually, I'm guessing that a copy of your add-on code (or something like that) is probably running exactly the same as if
the user was running if from their account.
Sometimes people contact me, asking why they are hitting the 100 recipient email limit on an free/consumer account
when they have my add-on installed.
Then I've got to explain to them that I don't work for Google, and payment for my add-on isn't payment for a GSuite account.
If, for example, you gave the user a copy of your add-on code, and they ran it from their account, and that code hit the quota limit,
then your add-on will hit the same quota limit.