I'm developing a service application with the purpose of keeping track of users presence in S4B. When the users presence change, the application will forward this event to consumers of the service, eg. other service apps. The strategy used so far is to log in as a system user and use event channels and subscribe to a distribution list. The application will be used in organizations with up to 10 000 users, so there will be a lot of events. Is there any limitations in UCWA that prevents me from doing this?
There is no documented limit, or experience provided by someone, to be used as reference, to reply your question.
From the documentation you can expect to run into some limitation, but what is the boundary is indeed lacking:
Forbidden 403 LimitExceeded The user has reached the maximum number of subscriptions that can be created.
and
Conflict 409 TooManyGroups The too many groups error.
From: PresenceSubscriptions resource
If you have previous experience with UCMA and its RemotePresenceView, you may know that there is a defined limit of subscribed users since when performance may degrade, and a limit on the maximum subscribers a user can have (defined by server policy), but there are experiences where UCMA has worked reliably for a scenario as you describe, and with high number of users.
See also this answer, related to UCMA: https://stackoverflow.com/a/16734244/2359892