As far as I know, there is no limit for ePO, there is just a question on how many do you need...
I currently have 200+ distributed repositories in ePO. In the past we even had more, probably close to 250.
I agree with the other posters, its all about how many you need. There are some ways of working it out in the PDF's you have.
I have one in each of my 4 main offices and 1 in a small office that has a terrible internet connection. Only about 7 in total in you count the ePO server and internet agent handler!
The limit is going to be how many you can realistically take care of rather than any physical limit of the product.
I have seen plenty of installations well in advance of 200 running just fine.
The fewer you have the easier it will be to administer though, so please think carefully about the requirements of the clients.
Also you need to remember that the more repos that you have, the more traffic that has to move from origination (typically ePO) to repo. I had an installation with 60+ repos, and the replication times become extremely crucial to ensure that all repos stay 'in sync'. Yes as rackroyd said, it is more the maintenance required to keep repos updated, patched, etc.... A single repo can easily take over 1k+ nodes. You just have to make sure that it is configured properly.