MAgirl75 - in answer to your question of how many is too many - that is a totally personal situation. I am on pregnancy number 16, with 12 first trimester miscarriages so far. After each miscarriage I said I could not do it again, but then after a bit found the strength to try again.
I have not been diagnosed with high killer cells as have not been tested for this, but have found a doctor willing to throw everything at this pregnancy, so I am on prednisolone, aspirin, heparin, as well as high dose folic acid. From my reading around the research, I read that mild clotting issues become more significant the more miscarriages you have (I had a positive TEG test while miscarrying, all other clotting tests normal), so I am pleased to be on the heparin and aspirin, I had an ANA+ test (this can be considered normal) but the prednisolone is for this. I can't do anymore this time.
I found that as I went through more miscarriages my coping methods changed each time. For the last miscarriage, I actually did the head in the sand approach and took no medications at all. I think that you can only do what is right for you. I did go for counselling after several miscarriages, and it was helpful to deal with the grief that had built up despite the pregnancies all being first trimester losses. Some miscarriages hit me harder than others, and this had no relation to how early or late they were, it was just the head space I was in at the time combined with how the miscarriage happened each time (reabsorbed/medical intervention/delayed).
Since you have been diagnosed with killer cells, I think that getting some of the research by Prof Quenby, and taking it to your appointment in Toronto might be helpful in putting forward your case for trying prednisolone.
In the news recently has been the reports of the of the woman having a baby after 20 miscarriages (following Shantala's treatment). It all depends on what you are willing to do. Only you can decide how much is too much for you. I hope that knowing that there are others that have been there too helps a little.