[Kanegasi]

It's better to understand if you format the table when typing it out.

Data={
    -- [mapID]
    [197]={
        -- [npcID] = XXX
        [91783]=1.94175,
        [95861]=1.94175,
        [100216]=1.94175,
    },
    [199]={
        [98370]=1.33333,
        [98368]=1.33333,
        [98366]=1.33333,
    },
}

Adding new keys to a table is as simple as creating any other variable.

Data[mapID] = {} -- this adds a new mapID with an empty table
Data[mapID] = Data[mapID] or {} -- this adds a new mapID with an empty table if that mapID doesn't already exist
Data[mapID][npcID] = number -- this adds a new npcID with a number (or changes the number of an existing npcID)

Here's what the creation code looks like if you were to add each thing individually.

Data = Data or {}
Data[197] = Data[197] or {}
Data[197][91783] = 1.94175
Data[197][95861] = 1.94175
Data[197][100216] = 1.94175
Data[199] = Data[199] or {}
Data[199][98370] = 1.33333
Data[199][98368] = 1.33333
Data[199][98366] = 1.33333

And finally, adding your example.

Data[999] = Data[999] or {}
Data[999][2222] = 1.1111