Answer
About 4,096,000 ids/s per worker. In review, anything from 3,410,000 to 4,920,000 counts as right.
The worked estimate
2¹² = 4,096 ids per millisecond. × 1,000 ms = 4,096,000 ids/s, about 4 million. When the sequence runs out, the generator waits for the next millisecond.
Numbers: Numbers to know.