Does Athens impose any limits on the number of users logging into a resource at the same time?

There are no concurrency limits imposed by Athens. However, some Content Providers limit the number of users from a single organisation  that are able to log into their service simultaneously. The number of users is often included in the subscription pricing. If a large number of your users are being blocked by concurrency limits, your organisation might want to consider purchasing an increase in the limit. If so, please contact the relevant Content Provider directly.