Personally, I've had 3.
Now here, I wanna make it clear what I mean by "Bone marrow transplant" in this post. I'm referring specifically to Allogeneic bone marrow transplants - one received from SOMEONE ELSE. It's possible to have an Autologous BMT too - in a procedure where your own blood stem cells are collected, so that they may be injected back into you after treatment. This is usually done to get very high doses, or treatments that would otherwise have killed off your vital bone marrow for good, into a patient, without killing them. The collected cells are stored and injected back into the patient after treatment has taken effect and remnants of the drugs have left the body.
An allogeneic transplant involves, on the patient end, first, some chemotherapy and radiation to the entire body (blood production, though often limited to bigger bones like your sternum or hip bone in adults, can occur in any bone in the body) to ablate, or kill off, your old, diseased stem cells. Less intense chemotherapy regimes can also be given, in diseases with lower agressiveness or in patients who can't take as much of a battering (my second transplant, due to all the drugs and radiation I'd had previously, was done using a "reduced intensity chemotherapy" regime. Many older patients are now able to get bone marrow transplants where they weren't before due to these regimes).
The major side effect of this step is that your blood production halts, or stops outright, meaning you will be much more likely to bleed and bruise in that period and be very susceptible to infections - which are even more life threatening as you don't have white cells to kill them off. Many of the drugs also have side effects of chemos (they often are chemos), and the radiation itself can cause severe mouth ulcers in some. This in itself makes the procedure very risky.
And if you catch GVHD fast, and do as recommended by transplant hospitals, and rush to emergency, or call up your doctor immediately to initiate treatment in the critical periods (up to 3 months after a transplant, or when you feel classic symptoms of GVHD like fevers, gut pain/diarrhoea, skin prickling/redness/rashes or eye dryness. It can affect many other organs in different ways too) - it is often gotten on top of!
There is a chance that you'll need to have surgery to remove actual bone marrow from your bones as well. Though it is unlikely nowadays, the procedure is done under general anesthetic meaning you won't feel pain from the operation, and is very safe too.