How long is a piece of string? Low voltage is an AC voltage of between 50 and 1000 volt so one would consider 230 volt as being low voltage so why would you need any transformer?
Extra low voltage is below 50 volt and often this means 12 volt as that is maximum allowed for some locations.
Most separated extra low voltage (SELV) lamps are supplied from inverters rather than transformers so there is a min and max wattage. Using 8 LED lamps likely they would run off one unit. But as you move to higher wattage lamps then it is a case of doing ones sums and you could be using 8 x 10W or 8 x 50W and there is no way one can answer your question.
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