To clarify, you want an external number to determine the Max value of a slider automatically?
If so, I don’t think there is a current node for that. The way I would approach it right now, and you might be able to create your own custom node to make the code a little neater is in the image below. Essentially, you use a slider with a range 0 - 1 and then multiple the list length. It is a bit messy, but it works…
I tried Danny´s suggestion, and it actually works quit well. I couldn´t figure out Dimitar´s version, since I have close to none experience with python. But it looks good as well.
Anyway, Danny´s idea works well for me at the moment, so I will stick to it for now.