When can we say - a couple of vs. many?
There are 3 ~ 4 (not counted, so can not be exact) tables in the restaurant - we say, there are a couple of tables in the restaurant.
There are ~ 100 seats in the theater, we don't say, there are a couple of seats in the theater, right?
So where is the cutoff for using a couple of, as opposed to, many?
Thanks
ps. can we use "a couple of" in writing (formal)?