1. They both refer to the number of leeches that are applied at one time - sually between 6 and 12. Does anyone know how much blood would be drawn off per leech?
A large leech will imbibe about 3 ml (0.003 liter) of blood. The wound will keep bleeding because leech saliva includes an anticoagulant.
2. After one delivery (and subsequent leeches) she developed some sort of infection on her leg, which apparently continued to cause trouble fornearly a year afterwards. the problem seems to be related to the leech bite - do leeches carry any infections that could cause this sort of problem?
No, leeches do not usually carry any infections. Those used in medicine are screened.
3. Did doctors re-use the same leeches? Because if so, surely there must have been a serious danger of cross-infection from one patient to the next?
To the best of my knowledge, leeches are not reused. They require up to six months to digest a large meal, hence reuse is not practical.