How do I format numbers such as 10,000 as 10K?

There is no pre-written function for this. Some tips:

• If n is a number greater than or equal to 1, math.ceil(math.log(n+1)/math.log(10)) will tell you how many digits the number has (ignoring the decimal). If this comes out to be 4, you could use "K" for instance.
• You could repeatedly divide the number by 1000 while it's >= 1000 and keep track of how many times you do this
• You can use string.format to automatically round to the nearest 0.1: string.format("%.1f", 12.37) produces 12.4
• You can use a table to choose which letter to use:

local letter = {
    [0] = "",
    [1] = "K",
    [2] = "M",
    --etc
}
local n = 100923
local numGroups = 0
local n2 = n
while n2 >= 1000 do
    n2 = n2 / 1000
    numGroups = numGroups + 1
end
print(string.format("%.1f%s", n/1000^numGroups, letter[numGroups]))