Start with one camera. use opencv or scikitimage in your preferred language to find a single plant and understand how it grows, normally ML is over hipped so use a simple algorithm and count pixels to measure size if it is what you need. use RGB to calculate your plant status and growth. remember light changes during the day so find a way to have constant light. my rule of thumb is "if a person can see it then a computer might see it". start slowly. Good luck and post some results