Usually, none. Looking at statistics from the United States, for which the most complete records exist, only about 2% of tornadoes result in human fatalities. Among killer tornadoes, most only kill one person.
usually hundreds if it is a minor tornado but if it is major, usually thousands of people.
No. A tornado is a localized event, usually not affecting more than a town or two. A tornado may lead to people leaving a town, or sometimes a tornado-prone region, but not usually a country. For example, many people left the small town of Greensburg, Kansas after most of it was destroyed by a tornado in 2007, but they all stayed in the United States.
The Greensburg, Kansas tornado of 2007 Killed 12 people and injured 63.
The Waco, Texas tornado killed 114 people.
The most significant effect on a natural habitat would be the destruction of trees. Even a tornado that is not particularly intense can snap and uproot many trees. The area affected is usually not very large, but it can take decades to fully recover.
28 people were killed in the Regina Tornado in 1912.
The Daultapur-Saturia tornado of 1989 killed an estimated 1,300 people.
There were no tornado fatalities in Florida in 2010.
The Tri-State tornado killed 606 people in Illinois.
On the date of April 26, 1989, the Bangladesh tornado killed 1300 people.
The Jarrell, Texas tornado of 1997 injured 12 people and killed 27.
The Waco, Texas tornado of May 11, 1953 killed 114 people, tying it with the 1902 Goliad tornado as the deadliest tornado in texas history.