Remaining MixPlay Buttons
I'd like for the DevLab to tell me how many buttons I have l left that can be placed. Just a number somewhere on screen that says you can still place # buttons.
As a developer working with the Mixer API, I can tell you, that there is no effective limit on the number of controls in a scene. You can, and this will work, add as many controls as you want, have invisible controls, controls of any type, and even controls above one another.
Due to the way this works, you can't really have what you are asking for.
Since you were however talking about buttons, and it isn't far to conclude that you are using the default controls, there are some guidelines you should follow about their minimum size (6 grid elements wide and 4 high), which the designer in the Dev Lab adheres to (an application can always make smaller buttons, though).
There are also three grid sizes, for desktop, tablet and mobile, which have different sizes (80x20, 45x25 and 30x40, respectively), and not all of them would fit the same amount of buttons, even at their smallest possible size. Simple math should tell you that. Also note, that due to the differing aspect ratios, depending on button size, some space on a scene may go unused.
The issue of how much space you have left to place controls, is then further complicated by their label text. Depending on what you want to write on there, buttons have to be a certain size to ensure the text is legible.
Last but not least, you can also add as many scenes and participant groups as you want, and assign your users to them at the push of a button, giving you all the room you could possibly need. The possibilities are limitless, and the idea to attempt to give you an estimate on how many controls you have left to place is so complex and error prone, that it would be an exercise in futility.