Are there any old manuscript of Hadiths books?
Yes many. I know when you buy copies of the hadith collections they contain pictures of them. I dont know any website links however on this.
Do a google search on manuscripts you may find them.
Is there any definite number of Hadith?
Logic dictates yes. As you cannot have q chain of Narration from now to the prophet saw that would be like 28+ preople.
Now new hadith collections can be made. E.g Sahih Abu dawud by Skeikh Al-Albani which is all the authentic hadith from Sunan Abu Dawud. But none of these Hadith are 'new'.
There are many hadith collections some authentic some not. So hadith always need to be graded.
You can check grades here
www.sunnah.comAs for the claim hadith can be added. This completely false and a lie. Yes hadith can be fabricated but no authentic hadith can just be made up or added to like Sahih Bukhari Etc.
As for saying that the hadith of tall building being "added". Complete lies its found in many collections of hadith e.g sunan an nasai , ibn majah etc.
Simply say bring your proof for your claim. Show me its been added etc.
As for hadith I dont know lots about the science. If your interested then read :
http://www.islamic-awareness.org/Hadith/And Allah Knows Best.