re your q's
"How many can I keep" is a very logical first question, and one asked by many who come on our pig courses.
Like many things piggy - start at the other end of the process - so a better question would be "what am I going to do with x pigs when they are ready?"
You could certainly keep quite a few on 40m*90m, but having say 8 pigs ready at the same time will give you an awful lot of meat. Butchers and abattoirs might buy finished pigs from you, but most won't (they have existing sources, and are usually unkeen to buy from unknowns) and you'll almost certainly make a loss. At livestock markets you'll lose even more -they sell to abattoirs and butchers, and selling to the public from home requires time and effort. Selling at Farmers Market's requires even more dedication.
So "toe in the water" approach is consumption for yourself and family/friends. I'd recommend you start with 2-3, and see how you get on. If you want larger numbers, then stagger purchase (say two this month, two a month or two later) so you get a production line going.
Breed wise - as others have said there is no right answer - if there was they'd be only one breed of pig! Lop ears (ear going down, rather than prick ears who have more sight) are generally easier, so we'd recommend them as a good start, but some have had say Tamworths (a prick eared breed) and got on fine - although we know some who have struggled and general feedback concurs with our practical experience that lop ears are an ideal first time pig. And as others have said - don't rule our crossbred pigs.
Best place to get them from? - we know several farmers who have premises made of tin and string and the whole place is very untidy, but have fantastic livestock and welfare, and have been to others where we have walked away from posh premises due to poor pig conditions. So don't be put off by first impressions, but DO look carefully around to see how all their stock is looked after. Preferably see them at 3-4 weeks - that way you can assess, choose and put down a deposit so you know what you are getting at 8 weeks. Don't be afraid to walk away if you get that feeling - you can always pretend to get that urgent call on the mobile and need to leave. Bottom line is ask around. if you're near us (Sussex) - drop us an email - if we don't have we can probably put you onto someone who we trust. If not ask on this forum, there are pig keepers from all over the UK on here, and they can recommend for your area and breed.
Finally get a good pig book (assuming you're not coming on one of our courses, where you get one free), we do one online (
http://www.oaklandspigs.co.uk/perfect-pigs-book/), but others are available, for instance the Haynes Pig manual is written by another TAS member.
And enjoy your new venture !