How many arguemnts are there in reduce method in lightning component.

Hi,
<lightning:input aura:id=”field” label=”Last name” placeholder=”Last name” required=”true” />
<lightning:input aura:id=”field” label=”First name” placeholder=”First name” required=”true” />
<lightning:input aura:id=”field” label=”Email” placeholder=”Email” required=”true” />
<lightning:input aura:id=”field” label=”Phone” placeholder=”Phone” required=”true” />