MongoDB: Mongoimport server selection error , connection refused, too many files open

Trying to insert several JSON files to MongoDB collections using shell script as following,

#!/bin/bash

NUM=50000
for ((i=o;i<NUM;i++))
do
    mongoimport --host localhost --port 27018 -u 'admin' -p 'password' --authenticationDatabase 'admin' -d random_test -c tri_${i} /home/test/json_files/json_${i}.csv --jsonArray
done  

after several successful adding, these errors were shown on terminal

Failed: connection(localhost:27017[-3]), incomplete read of message header: EOF
error connecting to host: could not connect to server: 
server selection error: server selection timeout, 
current topology: { Type: Single, Servers: 
[{ Addr: localhost:27017, Type: Unknown, 
State: Connected, Average RTT: 0, Last error: connection() : 
dial tcp [::1]:27017: connect: connection refused }, ] }

And below the eoor messages from mongo.log, that said too many open files, can I somehow limit the thread number? or what should I do to fix it?? Thanks a lot!

2020-07-21T11:13:33.613+0200 E  STORAGE  [conn971] WiredTiger error (24) [1595322813:613873][53971:0x7f7c8d228700], WT_SESSION.create: __posix_directory_sync, 151: /home/mongodb/bin/data/db/index-969--7295385362343345274.wt: directory-sync: Too many open files Raw: [1595322813:613873][53971:0x7f7c8d228700], WT_SESSION.create: __posix_directory_sync, 151: /home/mongodb/bin/data/db/index-969--7295385362343345274.wt: directory-sync: Too many open files
2020-07-21T11:13:33.613+0200 E  STORAGE  [conn971] WiredTiger error (-31804) [1595322813:613892][53971:0x7f7c8d228700], WT_SESSION.create: __wt_panic, 490: the process must exit and restart: WT_PANIC: WiredTiger library panic Raw: [1595322813:613892][53971:0x7f7c8d228700], WT_SESSION.create: __wt_panic, 490: the process must exit and restart: WT_PANIC: WiredTiger library panic
2020-07-21T11:13:33.613+0200 F  -        [conn971] Fatal Assertion 50853 at src/mongo/db/storage/wiredtiger/wiredtiger_util.cpp 414
2020-07-21T11:13:33.613+0200 F  -        [conn971]

***aborting after fassert() failure