According to a post on Reddit Trump has held events with as many as 31,000 participants. (Note that this list only includes campaign speaking engagements and not fundraisers or visits with government and corporate leaders.)
When comparing the Trump campaign with the Clinton campaign since the conventions in late July the results are staggering. Donald Trump held 31 campaign events in August. More than 198,000 people participated at these events in August and for 7 of these events (Columbus, Harrisburg, Ashburn, Wilmington, Abingdon, West Bend and Akron) Trump had overflow crowds.
Donald Trump has materially outpaced Hillary Clinton in rallies and in attendance at campaign events since the conventions in late July. Trump held triple the number of events that Clinton held (32 – 11) in August and had 20 times the number of participants. From August 17th through the 31st alone Trump campaigned in front of 88,000 at his events to only 400 (yes 400) for Hillary at her one event since the 17th!