Postgresql: Get pg_stat_statments.calls but per PID
pg_stat_statement cannot produce this kind of information.

You'd have to set log_statement = all, then you get all these statements (and all others) in the log. Make sure that you have %p in your log_line_prefix.

Be careful: logging all statements can bring a busy server down. In PostgreSQL v12, you can set log_transaction_sample_rate to log only a certain sample of transactions, then you can estimate the total count.