I'd like to give orange flavor and smell to my batch. When do you think is the best time to add it? Is it best to add orange while the wort is boiling or add it to primary fermentation? Thank you for your advices.
Common practice is to add it to the last few mins of the boil to sanitise it. Fruits (especially peels I belief) contain yeasts of their own so adding them to the primary without proper sanitation could end in unwanted results. Another option is to soak them in some solution, e.g. vodka, first prior to adding them in the primary.
Use only the orange zest, and put it into the hot wort when the boil is complete (at "flameout"). Use only the skin of the orange that is coloured, not the white underneath (the "pith").
Bitter oranges like Seville are typically used for better flavouring, but you can use everyday oranges too. For a first go, I would use more rather than less, as fermentation (or whatever) drives the flavour off. Say at least the zest of 4 oranges in a 20 litre batch. But 8 would not be out of the question either.
Failing that, you can always put it in at the end-stages of fermentation, but I have no experience in this with oranges. This is typically recommended for fruits with a less robust flavour.