John Adams (#2) did not own slaves during his lifetime.
Thomas Jefferson (#3) held more than 600 slaves for the purpose of his plantation at Monticello. He had several children with at least one of his female slaves (Sally Hemings). Jefferson did not free the vast majority of his servants upon his death - that being in direct conflict of his lifelong narrative of having been opposed to the institution both during his time in Virginia state politics and in federal office. To be fair, he would for example reject William Henry Harrison's advances [see further below], so maybe it wasn't all a lie.
James Madison (#4), father of the Three-Fifths-Compromise, held more than 100 slaves. One of his slaves, Paul Jennings, served him during his presidency and wrote a fascinating account of life in the White House, among the first of its kind. Madison failed to free his slaves in his will.
James Monroe (#5) owned between 50 and 100 slaves. Monroe is the head designer of the country of Liberia, a destination for freed U.S. slaves and the longest-standing independent country in Africa. The capital of Liberia, Monrovia, is named after him. Politically, he was more on the anti-slavery side of things, although he was a committed Anti-Federalist and states' rights advocate. He blamed slavery on the UK, identifying it as a British-made evil from colonial times - that is in hindsight in direct contrast of the British abolition of slavery in 1833 (most of the empire) and 1843 (removing the remaining exceptions), long before the United States, but Madison was in office while the British Empire still practiced slavery, especially in India.
John Quincy Adams (#6) did not own slaves during his lifetime.
Andrew Jackson (#7) owned more than 100 slaves. Despite being a "champion of the little man" during his political campaigns, he failed to be of the same virtuous value to the people of color he owned as property. He did not free any of them upon his death. Jackson would be infamous not only for his ruthless treatment of slaves (both politically and personally), but especially of Native Americans, with Jackson being the main mastermind of the "Trail of Tears".
Martin Van Buren (#8) is the first slaveholding President who did not own slaves during his time in office. The only slave Van Buren ever owned, a man by the name of Tom, manage to flee captivity in 1814 and was later found in Massachusetts. Van Buren did not take him back and did not hold slaves following that incident.
William Henry Harrison (#9) owned a small number of slaves, probably 11, most of whom he inherited from his father. He tried to get Congress to legalize slavery in the Indiana Territory while governor there during the Jefferson administration - Harrison's efforts did not succeed, and Indiana remained a non-slaveholding region and subsequently joined the union in the form of several free states.
John Tyler (#10) owned over 50 slaves. He personally expressed disdain at the practice, but remained in support of it politically. He did not free any of his slaves upon his death.
James K. Polk (#11) managed to win the power struggle against Van Buren inside the Democratic Party because of his friendliness to the institution of slavery. Polk, who owned between 20 and 30 slaves, imitated George Washington in his will (that went into effect with his death in 1849), promising his slaves their freedom after the death of his wife. His wife would however long outlive her husband and die in 1891, way after the abolition of the practice, and the slaves were freed before Polk intended.
Zachary Taylor (#12) owned more than 100 slaves and is the last President on our list to own them while in office. In spite of his personal ownership of slaves, he was unfriendly to attempts of turning newly acquired territories into slave states, and his premature death in office 1850 led to conspiracy theories by abolitionists accusing slavery supporters of poisoning him.
Millard Fillmore (#13), Franklin Pierce (#14), James Buchanan (#15) and Abraham Lincoln (#16) never were slaveholders in their lives.
Andrew Johnson (#17) had been Abraham Lincoln's vice president until Lincoln's assassination in 1865. He had been a committed supporter of James K. Polk during the Polk administration and had tried to dampen Lincoln's anti-slavery efforts. His 8 slaves had however been freed before his time in office.
Ulysses S. Grant (#18) was the last President in U.S. history to own slaves during his lifetime. He owned one slave, William Jones, and also had significant influence on four more, all of whom property of his wife Julia. Grant, in an at least somewhat satisfying ending note to this list, gave Mr Jones freedom instead of selling him to another slaveholder - despite being in financial trouble at the time.
Starting with Rutherford B. Hayes (#19), not a single U.S. President has ever been a slaveholder in their lifetime again.
The 13th Amendment to the U.S. Constitution was passed on December 18th, 1865, by proclamation of William H. Seward after its ratification process had been completed. This makes Warren G. Harding (#29), born on November 2nd, 1865, the last President in U.S. history born at a time when slavery was legal. Calvin Coolidge (#30) has been the first President whose entire lifespan was in a United States where slavery is abolished.
So that's that. To answer your question directly: Twelve. Twelve U.S. Presidents owned slaves during their lifetime.
Eight of them (Washington, Jefferson, Madison, Monroe, Jackson, Tyler, Polk, Taylor) were slave holders while in office.
Four of them (Van Buren, Harrison, Johnson, Grant) had let go of their slaves before taking the highest office in the land.
Not a single President has ever gone from being a non-slaveholder before and during office to suddenly being one after the fact.
All of these twelve men fall within the first 18 presidents, with John Adams (#2) and his son John Quincy Adams (#6) being the only two out of the first twelve presidents to never own slaves during their lives.