According to https://www.kernel.org/doc/Documentation/sysctl/vm.txt

max_map_count: the maximum number of memory map areas a process may have

The concept of “memory map areas” might be a bit abstract, but it can be made clearer with the help of pmap. Here is a C++ program, using mmap to manipulate some memory maps, then using pmap to show the effect. Basically, consecutive maps of the same type are automatically merged to be resource efficient. One can use vm.max_map_count to bump up the max number of maps a process can construct.

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#include <iostream>
#include <sys/mman.h>
#include <sys/types.h>
#include <unistd.h>

using namespace std;

static pid_t pid;

static void snapshot(int i) {
  constexpr auto len = 1024;
  char cmd[len];
  snprintf(cmd, len, "pmap %d > %d.txt", pid, i);
  system(cmd);
}

int main(void) {
  pid = ::getpid();

  puts("clean start");
  snapshot(1);

  puts("allocating 12k mem");
  constexpr auto size = 12 * 1024;
  auto m = mmap(nullptr, size, PROT_READ | PROT_WRITE,
                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  snapshot(2);

  puts("new map created");
  system("diff 1.txt 2.txt");

  puts("releasing the middle 4k mem");
  auto middle = (char*)m + 4 * 1024;
  munmap(middle, 4 * 1024);
  snapshot(3);

  puts("12k mapping broken into two 4k mapping");
  system("diff 2.txt 3.txt");

  puts("allocating the middle 4k mem again");
  mmap(middle, 4 * 1024, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS,
       -1, 0);
  snapshot(4);
  puts("back to one 12k mapping");
  system("diff 3.txt 4.txt");
  return 0;
}

The output of running the above program:

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> clang++ test.cc; ./a.out
clean start
allocating 12k mem
new map created
32a33
> 00007f7440430000     12K rw---   [ anon ]
42c43
<  total             5568K
---
>  total             5580K
releasing the middle 4k mem
12k mapping broken into two 4k mapping
33c33,34
< 00007f7440430000     12K rw---   [ anon ]
---
> 00007f7440430000      4K rw---   [ anon ]
> 00007f7440432000      4K rw---   [ anon ]
43c44
<  total             5580K
---
>  total             5576K
allocating the middle 4k mem again
back to one 12k mapping
33,34c33
< 00007f7440430000      4K rw---   [ anon ]
< 00007f7440432000      4K rw---   [ anon ]
---
> 00007f7440430000     12K rw---   [ anon ]
44c43
<  total             5576K
---
>  total             5580K