Since the release of FormAnalytics version 4.2.1, Matomo by default limits the number of form fields in a request to 2000 for better DB performance. This limit can be increased by adding below setting in your config/config.ini.php file.

[FormAnalytics]
max_no_of_form_fields_allowed={YOUR_DESIRED_NUMBER}

Previous FAQ: Increase the number of form start/submission requests allowed in total