Campaign staff says 3500, but that may be a bit inflated:
"The crowd bundled for the cold stood between two mill buildings to hear her all-but-inevitable announcement. It appeared fewer than 1,000 people people attended, but those who did were energetic."
eagletribune.com/news/warren-makes-presidential-campaign-official-in-lawrence/article_df0aacd6-2c8f-11e9-80d0-8fd9c7048f96.html