There are major differences in how the nations are guarding their borders. While some of them maintain regular army at their borders, some utilize special forces. EU is known to guard some of their exterior borders with the help of commercial company. In the United States, border control is the responsibility of the Department of Homeland Security.
The duty of guarding the border and preventing illegals from entering US (or criminals looking to flee) is shared by U.S. Customs and Border Protection, the U.S. Coast Guard and Immigration and Customs Enforcement. CBP role is primary inspection and enforcement, while Coast Guards are the primary interdiction component in both domestic and international waters. The role of Immigration and Customs Enforcement is largely investigative. The main agency that serves to prevent unwanted aliens from entering the US is United States Border Patrol. With over 21,000 agents, the U.S. Border Patrol is one of the largest law enforcement agencies in the United States. But it proved to be not enough, especially on Southwestern border. Patrols there are outnumbered by the perpetrators. The Border Patrol has lost 123 officers in the line of duty [from the 1904], half of them from hostile actions.
The National Guard of the United States is is a reserve military force and should be used in case of emergency. Trump proposal to send it for reinforcement of The Border Patrol is not the first time. National Guard was used by George W. Bush and Obama administration for exactly the same purpose.
Our Border Laws are very weak while those of Mexico & Canada are very strong. Congress must change these Obama era, and other, laws NOW! The Democrats stand in our way – they want people to pour into our country unchecked….CRIME! We will be taking strong action today.
— Donald J. Trump (@realDonaldTrump) April 4, 2018
As for know there is no data on how much of National Guardsmen will be serve on border. It is widely believed that the number will not exceed 1200.