Chomsky Normal Form (If all of its production rules are of the form):
A -> BC or
A -> a or
S -> $\varepsilon$
where A, B and C are nonterminal symbols, a is a terminal symbol (a symbol that represents a constant value), S is the start symbol, and $\varepsilon$ is the empty string. Also, neither B nor C may be the start symbol, and the third production rule can only appear if $\varepsilon$ is in L(G), namely, the language produced by the context-free grammar G.
Applying productions of the first form will increase the number of nonterminals from $k$ to $k + 1$, since you replace one nonterminal (-1) with two nonterminals (+2) for a net gain of +1 nonterminal. Since you start with one nonterminal, this means you need to do $l - 1$ productions of the first form. You then need $l$ more of the second form to convert the nonterminals to terminals, giving a total of $l + (l - 1) = 2l - 1$ productions.