No, past life regression is not dnagreous at all. It's a mediatiaon and as with any meditation nothing can harm you. It's sole purpose is for you to explore your previous lives and see if there is a pattern that emerges as to why you do things etc. There are professionals that will take you under but they can charge a small fortune and to be honest I don't think they get better results than what you can get if you do it yourself. and doing it on your own is free
It really depends on how good you are at meditating. You remember the past life meditations so there is no need to have someoen else there.
anyway, Brian Weiss actually has several guided mediations one of which does take you under for past lives. I am a massive fan of his stuff as you can tell. Have a look here:
http://www.bookdepository.com/Regressio ... 1401922351
http://www.bookdepository.com/Regressio ... 1401922337
http://www.bookdepository.com/Meditatio ... 1848500075
http://www.bookdepository.com/Journeys- ... 1401906610
Also, I am sure there are oodles on youtube which are free - you just need to look for them. And probably on torrent dwnload sites as well.
You will never find out about all of your past lives. There could be hundreds and not all of them would be relevant to your this time - you ahve already elarnt some of the lessons and moved on. You can ask your guides how many you have had though. You might be surprised. HTH ;)