Introduction to Treaty visas
Over the years the United States has signed treaties with most of the other countries in the world, in particular treaties of 'Friendship, Commerce and Navigation'. These treaties are designed to promote trade and investment between the USA and the other contracting state, thereby encouraging good relations and peace. 
Nationals (individuals or companies) of countries with such Treaties with the United States can obtain visas to work in the USA in order to develop and direct their investment in and/or trade with the USA. Such visas are called E-visas, and are of two types.

E-1 Visas - E1 visas are for individuals who reside in a treaty country and conduct a significant amount of trade with the U.S, either as a business owner or essential employee. Although there is no minimum amount of trade established to qualify, the level of trade must be high enough to qualify for this kind of visa. 

In addition, a treaty investor must invest a substantial amount of capital in a commercial enterprise in the US that they intend to develop and direct. Investment in a non-profit enterprise does not meet this requirement. Substantial capital is considered in relation to the total cost of purchasing or establishing the enterprise and must be sufficient to ensure the venture's successful operation. The business must also employ US workers and not solely benefit the investor and their family.

Employees of organizations wishing to apply for an E2 visa must be coming to the US in the capacity of an executive or supervisor that will be providing ultimate control and responsibility for the organization's operations. To qualify, the organization must be at least 50% owned by persons who have the nationality of the treaty country and the applicant.

E2 visa applicants may also be required to meet certain health and character requirements.
E-2 Visas - E2 visas are for individuals who have made significant investments in the U.S. and who reside in a treaty country. The investment must be made in an operating business and must have been made before petitioning for a visa. Like the E1 visa, there is no minimum amount of trade established to qualify, but the higher the trade level, the higher the chances that your visa will be approved. 

An E2 visa entitles the holder to live and work in the US temporarily. An E2 visa is usually granted for an initial period of 2 years and extensions may be granted. A treaty investor or employee may only work in the activity that was approved when the visa was granted. Any substantive changes in the investment or the organization's characteristics must continue to meet the E2 visa requirements and be approved by the USCIS.

Spouses and unmarried children under 21 years of age may apply for the applicable derivative visa (E2 visa) to join their spouse or parent in the US. Accompanying spouses and children are entitled to study in the US without obtaining a student visa and accompanying spouses may undertake employment with an approved employment authorization.

Based on publicly available information, it is not possible to know how many E-2 workers are in the country at any one time. The U.S. State Department only publicly tracks how many visas are issued per year, not how many people actually use the visas to enter the U.S. or how long they stay.

Fiscal Year              New E-2 visas issued
2013                              35,272
2012                              31,942
2011                              28,245
2010                              25,500
2009                              24,033