There actually are 19 Courtney series books. "When the Lion Feeds," published in 1964 and beginning just before the Zulu War of 1879, was the first, and it became the basis for a centuries' long history of the family going back to the late 17th century and forward to the late eighties. This list provides chronological order of publication and chronological order of the books.
https://www.bookseriesinorder.com/courtney/
They are all readable, though the latter works published with co-authors are not of the same quality as his earlier individual work. That is a technique used by of other aging popular franchise authors like Clive Cussler did before his death with his Dirk Pitt series.
Smith published another 40 novels in addition to the Courtney series. They are all readable and several are superb.