The Federal Trade Commission and the Department of Justice's Hart-Scott-Rodino Annual Report for Fiscal Year 2019 is available here. Appendix A to the report lists the number of second requests that each body makes each year. An average of 50 were made between 2010 and 2019, with no significant trend upwards or downwards, and there was an about even split between the two bodies.
The overall number of transactions that result in a second request is relatively low:
The number of transactions (mergers or acquisitions that raise concerns about their impact on competition) reported each year steadily increased over the course of the last decade: