1-I should add +30001 to the hex adress (6A) mentinonned on the datasheet to get the adress of the first register requested .
That depends on the ModBus library you plan to use. In most Arduino implementations I know you don't have to add the base register address because they use the transfered value directly which is zero based.
2-The total number of registers requested is 6 .
I don't know, based on the information you provided yet it's 2 but we are definitely missing a lot of information to answer that question exactly.