How many types of Roti are there?

Roti & chapatti are the same thing with a different name.

Both are made with only wheat flour (atta) & water - though some prefer to add a little oil, that's not really a part of the base recipe. If you add oil to the dough after formation, whilst rolling then folding you get a paratha instead. Milk & oil or ghee is going more towards a puri.

You can cook them in one of two ways. Dry-heat on a tawa or regular frying pan, then flame them, or just keep on the tawa. Personally I prefer flamed, but that's not too easy at home. The chefs I used to watch used huge tongs, like two wire table-tennis bats on a scissor to hold them over quite a wild flame. My gas range produces a very tame controlled flame by comparison & I have never been able to find those tongs anywhere.

The only other real difference, which may be area or even just family tradition related, is their thickness & pliability. My favourites growing up were always the ones that went soft & highly pliable after a few moments cooling; definitely best for picking up your food - we used to jokingly call them 'wash-leathers' but that was the type we liked & would return to establishments that could make them that way, rather then the dryer ones with a slight 'crisp' to them. Where I grew up most of the chefs were Bangladeshi, but I don't actually know if that variant is truly geographical.
I've moved away from that predominantly Bangladeshi-staffed area since my youth in Bradford & no-one around here (London) makes them that way, they're much more layered & dryer. Nowhere near as good imho.

Presumably, in the same way a plethora of various flavours, fillings & toppings have been added to the humble naan in the 40 years or so they're been widely available outside the sub-continent, you could add any other ingredient you like - but then it's not truly a roti any more.