How many dialects of Latin are there? Which dialect does Duolingo use?
Latin isn’t regarded as having any dialects per se (as in different forms of the same language spoken at the same time), but it has gone through several stages over history. For example, Vulgar Latin evolved from Archaic Latin and eventually split up and evolved into the modern Romance languages. The two kinds of Latin most people study are Classical Latin (the polished literary language used by the educated in the late Roman Republic and early Empire) and Church Latin (a written version developed in the Middle Ages which is still used by the Catholic Church and is an official language of the Vatican). Duolingo teaches Classical Latin with some updated spellings (for example, they use u, which was written as v by the Romans). The main difference between the two is some pronunciation conventions. If you learn one, you will have no problem with the other.