" 3.4.7 – The Length Operator
The length operator is denoted by the unary prefix operator #. The length of a string is its number of bytes (that is, the usual meaning of string length when each character is one byte).
A program can modify the behavior of the length operator for any value but strings through the __len metamethod (see §2.4).
Unless a __len metamethod is given, the length of a table t is only defined if the table is a sequence, that is, the set of its positive numeric keys is equal to {1..n} for some non-negative integer n. In that case, n is its length. Note that a table like
{10, 20, nil, 40}
is not a sequence, because it has the key 4 but does not have the key 3. (So, there is no n such that the set {1..n} is equal to the set of positive numeric keys of that table.) Note, however, that non-numeric keys do not interfere with whether a table is a sequence. "
