If you are using private proxies then 10 would be the minimum that you would need and most likely all that you would really need. So I agree with the above poster, 10-20 if they are private and not shared.
I don't know who you are looking at for a supplier but I highly recommend these 2 companies. I currently have subscriptions to both of them.
http://www.scrapeboxproxiesx.com/myprivateproxy/
http://www.scrapeboxproxiesx.com/buy-proxies/
Im buying from some guy on fiverr - i get 5 really fast private proxies for $5 a month
Right on, better purchase 2 gigs then since in my opinion you should be running around 10 proxies. $10/month is pretty good if they are fast, private and not recycled.