Greenplum was locked by more than 17 open connections(open transactions) in same time

I have 50 tables in greenplum database. I need load data to these tables by copy command. I opened connections consistently. 21'th connection locked with whole database. After that I performed copy command by libpq command for table in each thread(no commit, 20 transactions are opened). If connections loaders(50); for (int i = 1; i <= 50; i++) { PGLoader& loader = loaders[i]; loader.connectToDB("myhost", 5432, "myusr", "mypwd", "mydb"); loader.beginTransaction(); const std::string cmd = std::string("copy big_src.simple_tbl_") + std::to_string(i) + " from '/home/adb/document" + std::to_string(i) + ".csv' delimiter ',' csv header;"; //loader.performCommand(cmd); } for (int i = 1; i <= 50; i++) { PGLoader& loader = loaders[i]; loader.commitTransaction(); } } Copy command does not affect in here. Do you know there is exist some restrictions to work with 20 or more open transactions (connections) in same time?