It's written in small print under the screenshot of the "units widget" here [1]:
"Number of indexed records is capped at roughly 10% of annual search volume. See FAQ for more information."
The FAQ says:
"When using Annual Commitment pricing (minimum one year commitment and 1,200 units), each unit contains 1,000 searches x 12 = 12,000 searches during the year, and 100 x 12 = 1,200 maximum of records at any point in the year."