mail (normally uncount)
Eve native speakers don't know, mail is uncount. So we can only say "a piece of mail".
Mail is countable only if it means letter/parcel etc. delivered/collected at one secific time/period
It means mail.
every day (with a space in between!!) (adv)
"Many" can only be used for countable nouns
"Much" can only be used for uncountable nouns.
Determiners for count noun only
- a few, few, many
Determiners for uncount noun only
- a little, little, a great deal, a good deal
Determiners for both
- some, a lot of, lots of
To correct your statements:
I get a few pieces of mail every day.
I get a few letters every day.
I get too many letters every day.
I get several letters every day.
I get three letters every day.
There are too many cars in the street