Overview
The rules for soccer coaches depend on the individual league or organization. Generally one head coach is necessary for functions such as submitting a roster and participation in the coin toss. Other regulations are variable.
The limiting factor for the number of soccer coaches a team has is usually efficiency--not the rules. In other words, it might be perfectly legal to have 10 coaches on the sideline, but it would do more harm than good. Too many cooks, as they say, spoil the soup. That said, only one or two coaches are generally allowed to give sideline instruction during games.
FIFA
The International Federation of Association Football (FIFA) is the chief governing body of international world-class soccer. FIFA does not regulate the number of assistant coaches a team may have on the sideline, but the rules do stipulate that only one coach is authorized to give tactical instruction from the sideline during the course of a game. This is why you might see assistant coaches in whispered conversation with head coaches during the games: so that the head coach can relay his assistant's counsel to the players.
NCAA
The National Collegiate Athletic Association (NCAA) is the primary governing body of college sports. It follows most of the rules of FIFA, although there are some differences, such as the size and position of the coach's box or "technical area." The changes do not include any mention of the number of coaches; therefore, just as in FIFA, only one coach is permitted to give in-game tactical instructions.
NFHS
The National Federation of State High School Associations (NFHS) is the primary governing body of high school sports in the United States. Like the NCAA, the NFHS follows the rules of FIFA with few exceptions, and these exceptions do not include the number of coaches. Thus, like in FIFA rules, only one coach is authorized to give instruction from the sideline during games.
AYSO and Youth Soccer
The American Youth Soccer Organization (AYSO), like the NCAA and NFHS, subscribes to the rules of FIFA with a few exceptions. But for AYSO, these exceptions do mention the number of coaches. In AYSO, two coaches instead of one are allowed to give tactical instructions during the game. These two are intended as offensive and defensive coaches, but this particular use is not prescribed. The two coaches are free within the rules to give whatever advice they choose--except, notably, excessively negative or derisive comments, which are specially outlawed in AYSO rules. As in FIFA, NCAA and NFHS, the coaches are limited to an area at the sideline.
More Specific Rules
Local soccer organizations may have slightly altered rules regarding the number of coaches. It's best to check with your specific organization or association if you have questions.
Resources