PRO2PDF does not have a limit to the number of fields that may be mapped. The only limits are in your server’s PHP settings.
We have not tested any PDF forms with more than 700 fields. For that number, we recommend the following settings. max_execution_time = 600 max_input_time = 600 max_input_vars = 4000 memory_limit = 512M post_max_size = 256M session.gc_maxlifetime = 1440
Feel free to adjust your server’s settings to maximize the wait time to merge as many fields as possible.