ab is a tool for benchmarking web servers. It generate large amount of concurrent requests to a web site, simulating high traffic. This helps you identify how many visitors your web server able to handle.

ab -c 30 -n 10000 URL_OF_THE_SITE

Here we generate 30 concurrent requests. Total of 10K requests are made for the benchmarking session.

ab is part of Apache, you can find more details at

https://httpd.apache.org/docs/2.4/programs/ab.html

On Debian/Ubuntu, you can install ab with

apt install -y apache2-utils

Here is a sample benchmark result. Apache vs Nginx

boby@hon-pc-01:~$ ab -c 50 -n 1000 http://localhost:8080/
This is ApacheBench, Version 2.3 <$Revision: 1706008 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Completed 600 requests
Completed 700 requests
Completed 800 requests
Completed 900 requests
Completed 1000 requests
Finished 1000 requests


Server Software:        nginx/1.11.5
Server Hostname:        localhost
Server Port:            8080

Document Path:          /
Document Length:        21 bytes

Concurrency Level:      50
Time taken for tests:   0.075 seconds
Complete requests:      1000
Failed requests:        0
Total transferred:      252000 bytes
HTML transferred:       21000 bytes
Requests per second:    13292.75 [#/sec] (mean)
Time per request:       3.761 [ms] (mean)
Time per request:       0.075 [ms] (mean, across all concurrent requests)
Transfer rate:          3271.26 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    1   0.8      1       4
Processing:     1    3   1.4      2       7
Waiting:        0    2   1.5      2       7
Total:          2    4   1.2      3       7

Percentage of the requests served within a certain time (ms)
  50%      3
  66%      4
  75%      4
  80%      5
  90%      5
  95%      6
  98%      7
  99%      7
 100%      7 (longest request)
boby@hon-pc-01:~$ ab -c 50 -n 1000 http://localhost/
This is ApacheBench, Version 2.3 <$Revision: 1706008 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Completed 400 requests
Completed 500 requests
Completed 600 requests
Completed 700 requests
Completed 800 requests
Completed 900 requests
Completed 1000 requests
Finished 1000 requests


Server Software:        Apache/2.4.18
Server Hostname:        localhost
Server Port:            80

Document Path:          /
Document Length:        21 bytes

Concurrency Level:      50
Time taken for tests:   0.687 seconds
Complete requests:      1000
Failed requests:        0
Total transferred:      267000 bytes
HTML transferred:       21000 bytes
Requests per second:    1454.55 [#/sec] (mean)
Time per request:       34.375 [ms] (mean)
Time per request:       0.687 [ms] (mean, across all concurrent requests)
Transfer rate:          379.26 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.5      0       3
Processing:     2   34 129.6      4     601
Waiting:        2   34 128.8      4     600
Total:          2   34 130.0      4     602

Percentage of the requests served within a certain time (ms)
  50%      4
  66%      4
  75%      6
  80%      6
  90%      7
  95%    597
  98%    601
  99%    601
 100%    602 (longest request)
boby@hon-pc-01:~$ 

If you check Percentage of the requests served within a certain time (ms), you will see Nginx was able to service all requests under 7ms. With Apache, it take 597 ms or more for 95% of the 1000 request we made during the benchmark session.