Hi, Murray. The FTDNA yDNA haplotree changes pretty much monthly, sometimes semimonthly. When this question was posted back in September 2018, there were 16,361 branches on FTDNA's Y-Tree; three years later, on 11 Sep 2021, there were 47,785 branches; as of today, it's up to 67,153.
The mtDNA tree is a different animal. YFull--where I've also had my own yDNA and mtDNA analyzed--is a bit of a rogue element for mitochondria. Not necessarily correct or incorrect, just non-standard. FTDNA has stayed with (at the least for the most part) Phylotree, which is still on Build 17 from February 2016.
There are simultaneous efforts underway right now with a goal of improving the haplotree and updating the branches. In fact, two different entities are using the term "Mitotree" for the result: both FTDNA and work underway by Nicole Huber, Walther Parson, and Arne Dür in Europe, evidently using the forensic EMPOP database (https://empop.online/) as its repository of reference. The International Nucleotide Sequence Database Collaboration (INSDC, https://www.insdc.org/)--a joint initiative among the U.S.'s NCBI (part of the National Institutes of Health), the DNA Data Bank of Japan (DDBJ, and the EBML-EBI (the European Molecular Biology Laboratory's European Bioinformatics Institute)--has a stated position that scientific journals should, for published papers dealing with mtDNA, submit the full sequences to NCBI's GenBank, DDBJ, or ENA (the European Nucleotide Archive).
Part of the overall complexity with the mtDNA haplotree is that the mitochondrial DNA molecule is amazingly tiny. Unlike yDNA where we have over 23 million base pairs and their possible variants to work with--and with the Y-Tree being much cleaner in that variants tend to line up chronologically for us in ancestral/derived delineation--with mtDNA we're looking at only about 16,569 total base pairs, and it's more about the overall collection of variants. Not a one-to-one but a one-to-many. The same variant at the same locus might be included in over a half-dozen subclade designations, some of which might not even have the same basal, top-level clade.
Too, care has to be given in making sure that the designated haplogroups represent germline DNA, the DNA that's passed along in the ova...which for obvious reasons we don't typically examine. What we test with the typical cheek swab is somatic DNA, body DNA: we test a bunch of mitogenomes in epithelial cells that are some of the roughly 4 quadrillion mitochondria that are in our bodies at any given time...and they're constantly making copies of themselves because mitochondria have a replication half-life of as brief as 8-11 days for things like skin cells, and 20-30 days for long-lived cells like neurons. We don't directly test the germline mtDNA because they are already formed in the oocytes while the mother herself is still a fetus.
From experimental studies we know that over 61% of us carry mtDNA heteroplasmies, instances where two different mitogenomes are in one organism, even different genomes inside single cells. The reality is likely that very nearly all of us are heteroplasmic: no copy machine is good enough to replicate and replace 4 quadrillion items every four weeks or fewer and not have copy errors. That's the only way the mitochondrial DNA changes. And it's only when those replication "errors" become so prevalent that they're passed along into daughters, and then their daughters, that they begin to work their way into becoming germline stable. At FTDNA, for example, they consider concentrations of a minor allele--meaning the DNA "letter" that is the least common at a given position in a sample's DNA--of less than 20% to be a degree of heteroplasmy not worth identifying.
I'm rambling but, yep, the little mtDNA genome isn't straightforward to deal with in terms of haplogroups. But we definitely need an update that is going to be acceptable by consensus...meaning that, unlike the work YFull has been doing, the updated haplogroups and their specific variants are agreed to by all the major players, including forensic databases, academic institutions, and scientific journals.
At RootsTech 2023 last March, FTDNA announced that they would be focusing on updates to the mtDNA haplotree and hoping to provide additional tools of the sort they've recently added for yDNA. A more accurate TMRCA calculation for one would be very welcome, because the one in use is extremely broad and, in fact, overly optimistic when compared with the plethora of mtDNA research papers out there on germline mtDNA mutation rates. Last year, Roberta Estes posted about the Million Mito Project. That might be worth a read for some background. And every day that I see a new Group Project Administrator newsletter from FTDNA, I check to see if we have an update on current mtDNA efforts.