if investigator intercepts few monster at 1 street, he has to fight (or evade) all of them.

do the monsters receive any bonuses for their outnumber or investigator just fights then one by one?

Veross said:

No bonuses for the monsters : if there is more than one monster in an area, the investigator must evade/fight each monster in turn, in any order he chooses.

Agreed. One by one.