So it seems Stroth will contribute one volume, to go at the end, and we will have volume 11 (and maybe vol 12) of the main series before that. So 11+2+1 (or 12+2+1) volumes in total. [edit: the +2 is the Aschbacher–Smith work, the +1 is Stroth]
EDIT 9 October 2023
Commenter colt_browning points out below that Volume 10 is due for publication 26th December, and is now available for preorder: https://bookstore.ams.org/surv-40-10. The title is The Classification of the Finite Simple Groups, Number 10: Part V, Chapters 9–17: Theorem $C_6$ and Theorem $C^*_4$, Case A, with listed authors Capdeboscq, Gorenstein, Lyons and Solomon, and it's 570 pages long.
This book is the tenth in a series of volumes whose aim is to provide a complete proof of the classification theorem for the finite simple groups based on a fairly short and clearly enumerated set of background results. Specifically, this book completes our identification of the simple groups of bicharacteristic type begun in the ninth volume of the series (see Mathematical Surveys and Monographs, Volume 40.9). This is a fascinating set of simple groups which have properties in common with matrix groups (or, more generally, groups of Lie type) defined both over fields of characteristic 2 and over fields of characteristic 3. This set includes 11 of the celebrated 26 sporadic simple groups along with several of their large simple subgroups. Together with SURV/40.9, this volume provides the first unified treatment of this class of simple groups.
Total length of volumes 1–10 is 4511 pages, and Aschbacher and Smith's two volumes fill 1320 pages. Maybe another 1000–1500 pages to go? There's an old manuscript of Stroth from the late 90s that seems to cover the "uniqueness case" (first listed article on this page), which is what his volume will cover. That's 244 pages, but it's not clear how it relates to the draft of what will become the last volume of the published second generation proof.
