I am not sure how many is good or bad, but as long as you are not spamming or anything, you should be fine with 10 to 20. I know I have a total of about 12 I think between larger sites and my businesses and I have never had a problem.
I have many more than most and I have not had a problem with any of them. I only have one update a day on most of them, so I don't think they get flagged for being spammy which might help.
My dear friends IP never was, and never will be the issue in any serious business. 99 % of internet user have shared IP, that means your IP is used by hundred more other users at the same time. Quite often even when you pay extra for dedicated IP you share it with the others. Thus when you ban IP yo usually ban a group of people. There are other ways to identify the PC.
I dont think their is any ip limitation, but if you do spamming then your account will be banned by twitter.
due to subnets and masks - IP's are the main way. MAC addresses can also be used, but I believe you have to be directly connected for them to identify your MAC address.