Every number here is a visa the US State Department issued. It shows demand and how much the consulates process, not how many interview slots are open.
By nationality or by consulate
“Issued to nationals of India” counts Indian citizens wherever they applied. “Issued in India” counts what the US consulates in India issued, to anyone. The two differ, so each number says which one it is.
Last 12 months
The 12 months up to the latest month State Dept has published, compared with the 12 months before. Monthly reports arrive a few months late. Calendar years run January to December.
Student Visa Kit