UDA says sequence numbers can be implemented as an unsigned 32 bit integer:

"Event messages are tagged with an event key. A separate event key must be maintained by the publisher for each subscription to facilitate error detection (as explained below). The event key for a subscription is initialized to 0 when the publisher sends the initial event message. For each subsequent event message, the publisher increments the event key for a subscription, and includes that updated key in the event message. Any implementation of event keys should handle overflow and wrap the event key from 4294967295 back to 1 (not 0). Subscribers must also handle this special case when the next event key is not an increment of the previous key. Should be implemented as a 4 Byte (32 bit) unsigned integer."