Time to construct a GNBA for LTL formula
In general, logical formulae can be thought of as trees; inner nodes are operators and leaves are atomic propositions. Therefore, every formula consists of as many direct subformulae (that is on the first level) as its top-most operator's arity. For example,

$\qquad \varphi \land \psi$

has two direct subformulae $\varphi$ and $\psi$. This can be continued recursively and we see that

$\qquad \displaystyle |\operatorname{subf}(\varphi)| \leq \sum_{\circ \in \operatorname{op}(\varphi)} \operatorname{arity}(\circ)$,

where $\operatorname{op}(\varphi)$ is the multi-set of operators occurring in $\varphi$. Informally speaking, every operator contributes its operands to the set of subformulae. Note that we have $\leq$ and not $=$ because a subformula might occur multiple times.

In LTL, all operators have at most arity two, so $|\operatorname{subf}(\varphi)| \leq 2|\varphi|$ provided that $|.|$ counts (at least) operator occurrences.