How to do it...

1. Open your game mode's declaration and add a new property to the class:

UCLASS()
class CHAPTER_07_API AChapter_07GameModeBase : public AGameModeBase
{
    GENERATED_BODY()

public:
    virtual void BeginPlay() override;

    TArray<IMyInterface*> MyInterfaceInstances;
};

2. Add #include "MyInterface.h" to the header's include section:

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/GameModeBase.h"
#include "MyInterface.h"
#include "Chapter_07GameModeBase.generated.h"

3. Add the following within the game mode's BeginPlay implementation:

for (TActorIterator<AActor> It(GetWorld(), AActor::StaticClass()); 
     It; 
     ++It)
{
    AActor* Actor = *It;

    IMyInterface* MyInterfaceInstance = Cast<IMyInterface>(Actor);

    // If the pointer is valid, add it to the list
    if (MyInterfaceInstance)
    {
        MyInterfaceInstances.Add(MyInterfaceInstance);
    }
}

// Print out how many objects implement the interface
FString Message = FString::Printf(TEXT("%d actors implement the 
                              interface"), MyInterfaceInstances.Num());

GEngine->AddOnScreenDebugMessage(-1, 10, FColor::Red, Message);

4. Since we are using the TActorIterator class, we will need to add the following #include to the top of our GameModeBase class' implementation file:

#include "Chapter_07GameModeBase.h"
#include "MyInterface.h"
#include "SingleInterfaceActor.h"
#include "EngineUtils.h" // TActorIterator

5. If you haven't done so already, set the level's game mode override to your game mode, then drag a few instances of your custom Interface-implementing actor into the level.
6. When you play your level, a message should be printed on screen that indicates the number of instances of the interface that have been implemented in Actors in the level: