Is it Possible to Tell How Many Letters/Numbers a Value/String contains?

First:

To just get the length of a string str, you could use either #str or str:len() or string.len(str).

Let's say you wanted to count how many digits a string contains. We could count them:

function isDigit(s)
	-- lots of ways to do this:
	return tostring(s)
	return ("0123456789"):find(s)
	return s:match("%d")
	-- (in reality you would probably just put this line
	-- into that if -- unless you were doing this
	-- all over the place)
end



local str = "1apple2banana3"
local count = 0
for i = 1, #str do
	if isDigit(str:sub(i, i)) then
		count = count + 1
	end
end
print(count) -- 3

This is a bit long and annoying, though.

Observation: we are counting 1 for each digit character, and ignoring all other characters. Thus, if we had digits string which was only the digits of str, then we would just have to find its length:

local str = "1apple2banana3"
local digits = str:gsub("%D", "")
-- global substitution (find&replace)
-- of all not digits ("%D") with nothing ("").
print(#digits) -- 3

You can do this with more complicated patterns, too. In general it might sometimes be easier to write what is something rather than what is not (e.g. using "%d" over "%D"). In that case we just need to subtract the difference:

local str = "1apple2banana3"
local notDigits = str:gsub("%d", "")

-- #digits  +  #notDigits  =  #str
-- #digits  =  #str  -  #notDigits
print( #str - #notDigits ) -- 3

For other sorts of matches, look up information about string patterns. "%a" is letters, "%A" is not letters, etc.