I would get 2 now, then in a year another 2. This will give you a rolling mixed age group which should mean you don't risk a lone rat and you get the added bonus of younger rats keeping older rats acting young for longer.
Personally, I think 3 is a good number. Initially, we started with a trio ourselves and we enjoyed having 3 siblings from the get go. The downside is that when they get older, you may have to medically treat or lose 3 elderly rats simultaneously or in close consession (which is not fun). The positive aspect of having a trio is that when one rat dies, the other will still have the company of a sibling and you won't have to worry about introductions with a strange rat. In the end it really is up to you as 3 rats are no more difficult to manage then 2. In terms of where to get rats (not sure if that is what you were asking), I suggest a rescue or reputable, ethical breeder in your area as the first choice for obtaining a pet rat. If you throw out your general location, we can refer you to any shelters, rescues or breeders that we know of in your area.
Three is a good number I started off with pairs but now I have a trio it's great! Someone always wants to play and you really get to see group rat dynamics. I don't think it's any difference in terms of cleaning or money either.
I cannot help you with the location as I am more familiar with US breeders and rescues but there are quite a lot of members who reside in England who should be able to help you out. You can also try browsing the adoption section.
Your not too far from me, i'm just across the other side of the penines.
I would recommend coming to the NERS gosforth rat show in decemeber (info here http://www.neratsociety.co.uk/html/nextshow.shtml ). Its your local rat club and a fair few local breeders will be coming along as well as me (i'm a regular lol) and its a good opotunity to meet different rats and get a feel for what you want as well as chat to breeders. In terms of breeders who are actively breeding near newcastle your closest is probably stilly rats, i think they are still breeding, i havent caught up with them for a while.
Dont let distance put you off though, rats travel well and often even if your struggling for transport yourself often rat trains can be arranged. We've had a lot of rats from myself and my breeding partners (not what it sounds like lol) over on this side of the penines go and live over in newcastle. In fact a lot of our applicaitons come from there. The best thing to do is to meet a few rats from the different lines and get a feel for what temprement suits you best. So our rats are generally very active, naughty and in your face, they can be a bit too much full on for some people. Other breeders tend to aim for calm cuddly rats or other temprements. Then theres what varieties you are most (or least) keen on if that matters to you.
Thank you for all the replies. I had one of my dog in the vets for her vaccinations this morning. How weird is this - There was someone in with a rat. I told them I was getting some rats soon to. Evidently she breeds them. The rat she had with her she had bred her self and it eye didn't open properly. I have taken her details. So as long as they are well looked after then I have some where to get them. I spend a lot of time at the vets. As up until we had her put down recently we her 2 dogs with heart conditions. I never see any one with rats until I am looking to by. some.
Just be careful, she may be a wonderful breeder, but there are a lot of breeders out there that arent great or dont really breed for temprement or health, i.e. keeping records and such. The fact she had a rat in the vets is definitly better than some i've come across. Just make sure you ask lots of questions and get to know her and the parents, as well as why and how she breeds. You can get lovely rats from many different breeders but you can also get rats which are poorly bred and go on to get health issues early on. I'd also expect her to ask lots of questions about you and how you intend to keep them as well. Its a sign of a good breeder when they care how you plan to look after the rats, and most importantly keep in touch.