[Calisthenics]

An example using FileManager but it has to read once to retrieve how many files exist and a second to select the random one:

pawn Код:

new dir: dHandle, item[40], type, counter, random_file;
    
dHandle = dir_open("path_here");

while (dir_list(dHandle, item, type))
{
    if(type == FM_FILE) counter++;
}
        
dir_close(dHandle);

random_file = random(counter);
counter = 0;
dHandle = dir_open("path_here");

while (dir_list(dHandle, item, type))
{
    if(type == FM_FILE && counter++ == random_file) break;
}

dir_close(dHandle);
printf("random file: \"%s\"", item);

If you know the number of files will not exceed a certain amount, an array to store the file names and avoid the second loop is also a choice.