Does exalted stack when multiple creatures with exalted are on the battlefield?

Exalted is a triggered ability that reads: "Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn" (CR 702.90a). The key word is each instance triggers independently.

When you declare exactly one attacker, every exalted ability you control — whether from one creature or several — triggers simultaneously. Each trigger resolves separately and adds its own +1/+1 bonus to the attacking creature (CR 603.2, 603.3). There is no rule limiting how many times the bonus can apply.

Importantly, the creatures with exalted do not need to be attacking. They simply need to be under your control when the single attacker is declared. A creature sitting on the bench can still grant its exalted bonus to the lone attacker.

Example: You control three creatures: Sublime Archangel, a Duty-Bound Dead, and a Sigiled Paladin. You attack with only Sigiled Paladin. All three exalted abilities trigger (Sublime Archangel also grants exalted to your other creatures, but the core rule is the same). Sigiled Paladin gets +3/+3 until end of turn from the three separate triggers resolving.

Because each trigger is its own separate event, there is no interaction that prevents stacking — opponents cannot respond "between" triggers in a way that stops the cumulative bonus once the triggers are all on the stack.