For any number of players
To officially start the game someone calls out loud, ZERO. The players can be anywhere they like in the space. Moving is optional.
After a time (or very quickly) someone calls out loud, ONE. At this call each player must to look to another player and you must never loose sight of the person you have chosen to look at. You need to see this person the whole time but it can also be just a part the body that you see. There is no need to exchange gazes or that the same two people must look at each other. (But this is possible). Moving is necessary only to allow you to keep seeing this person.
After a time (or very quickly) someone calls out loud, TWO. At this call each player keeps seeing the first person they have chosen and adds another. Again you only need to see part of the body of the people you have chosen and there is no need to exchange gazes. But you must be able to see the two people you have chosen at the same time - all the time. To look at one person and then the other is not correct. You must never loose sight of the people you have chosen to look at. You may have to move to keep seeing these two people.