As with men, who wants to serve in ground combat and qualifies, depends on the individual. Right now, men volunteer for ground combat positions. One way to estimate how many women would be interested, is to survey serving female Soldiers and Marines, including officers. Of those, you could establish who is qualified.

The only way to know how many of these women could actually complete the training programs and perform well in the field, is to do it. Even then, these women would only prove their individual ability and determination.

The more immediate issue is the colocation (proximity) versus collocation (proximity and interdependence) interpretation of the current Army policy restrictions. Compare what the Marines are doing. My recommendation is to do a serious review. However, this is something the active duty force has to figure out.

As to how many it takes to make "it worthwhile", that depends on how you define "worthwhile" and the standards. Just how many "accommodations" are really necessary, and how many are the result of paternalism? When the chips are really down, like with the Soviets in WW II, you do what it takes to get the job done.

Which comes back to the larger issue of peacetime versus wartime mobilization. For military manpower planners, defining "worthwhile" is a function of demand and supply.

In peacetime, the vast majority of American men are not interested in military service, let alone the infantry. In wartime, it depends on the cause. If the cause is compelling, men have volunteered in droves. During both WW I and WW II, the War Dept. eventually prohibited volunteering so that men had enter the military through the Selective Service process.

In these mass mobilizations, Selective Service was used as a way of scientifically managing manpower, while ensuring enough men were available to work in the mobilized economy. Who fought in the infantry, artillery, aviation, Navy, or whatever capacity, was determined by the services, needs at a given time, and individual attributes. A man made his preferences known, but his aptitude scores, education, and theater needs drove the assignment process.

Since these wars were predominately fought with conventional forces, the major requirement was for a large number of ground forces. Another driver was the requirement for men sufficiently intelligent and educated that could operate and maintain the benefits of technology. The mechanization of the armed forces fundamentally altered the manpower equation.

This is not to say the nature of war has changed, but technology certainly influences the conduct.

Despite the limited warfare in Iraq and Afghanistan, the Total Force is smaller today than during the Gulf War. By the end of Desert Storm, there were over 500,000 forces in theater. In Iraq, the number of forces is roughly a third of that. Yet the Army, the service most heavily invested in Iraq, has had to lower its standards to barely meet recruiting goals.

The difference is the cause. While some men are motivated to join solely by a desire to prove their masculinity, the reality is that most young American males are sitting on the sidelines. If the Taliban invaded the country, women might push men out of the way to fight. In America, cause greatly influences the "worthwhile" analysis.