There were 15 slave states: Florida, Georgia, North and South Carolina, Alabama, Mississippi, Louisiana, Texas, Arkansas, Missouri, Tennessee, Kentucky, Virginia, Maryland, and Delaware.
The constitutional state governments of 11 of these passed secession ordinances and were indisputably part of the Confederacy. The Confederacy recognized the outlaw factions who passed secession ordinances in Missouri and Kentucky. The constitutional governments of these states never seceded though. The slave states of Delaware and Maryland remained in the Union and this was never seriously in question.