Irritating age/timeline/population questions

[Charmian]

No: HPFGUIDX 8747

I know many people find this subject annoying, so I've warned you all 
in the header. 

Timelines/ages
Where do the following stand in relation to James Potter's, Sirius, 
et. al.'s generation?
1)Bartemius Crouch Jr. 
2)Rita Skeeter
3)Bertha Jorkins (IIRC Sirius mentions her as either being a few 
years ahead or behind them at Hogwarts, but not having GoF with me 
right now, I can't confirm)

Since we can't even agree as to how many students there are per class 
at Hogwarts, this question is mostly meaningless, but have you 
wondered how many of the students at Hogwarts are from muggle 
families? Obviously, since they are educated to live in the wizard 
world, one assumes that they would join it at graduation and 
assimilate into it. If this has been going on for a long time, why 
are there so many pure-bloods left? 

OWLs and NEWTS:  Someone told me that they're the equivalents of the 
O and A levels, which are kind of like the AP or IB exams. How many 
of these can one get (because they did give the number that Crouch 
Jr. and Percy, I believe, got)?

Charmian