Quick answer: There is no magic number. A street sign should only have what people can read in the time they have to look at it. Usually that means one offer, one name, and one action. Cut everything else.
Think in seconds, not inches
A big sign does not mean more room for words. A big sign passed fast may hold less text than a small sign someone stands and reads.
Make one line the star
The first line should give the reason to look. The next part says who you are. Add an action word only if the sign still stays clear.
Keep details off the sign
Terms, menus, lists, and addresses belong somewhere a person can stop and read, not on the street sign. The sign's job is to get them to that next step.
Test it for real
Print the sign at real size. Step back. Give yourself a few seconds. Ask someone new what they got from it. Change the sign based on their answer.