I need to make the number 50 to stand up right for a cake next week.

I am not sure which is the best method. SHould I make it out of fondant / gumpaste, colorflow, chocolate or RI.

Any suggestions?

You can make a really stylish curly scrolly stand up 50 with GP...after you roll it out and shape it, lay it flat to dry, and insert some skewers into the bottoms at the points where they will sit on the cake. That should do it.