In order to increment apex:variable inside the apex:pageBlockTable, we have to include it inside apex:column in apex:pageBlockTable.

 <apex:page controller=”sample”>
    <apex:pageBlock >
    <apex:variable var=”i” value=”{!0}”/>
        <apex:pageBlockTable value=”{!acnt}” var=”acc”>
            <apex:column >
                {!i}
                <apex:variable var=”i” value=”{!i+1}”/>
            </apex:column>
            <apex:column value=”{!acc.Name}” rendered=”{! IF((mod(i,2)) == 0, true, false)}” style=”background:pink;”/>
            <apex:column rendered=”{! IF((mod(i,2)) == 0, true, false)}” style=”background:pink;” value=”{!acc.Rating}”/>                                
            <apex:column value=”{!acc.Name}” rendered=”{! IF((mod(i,2)) == 1, true, false)}” style=”background:yellow;”/>
            <apex:column rendered=”{! IF((mod(i,2)) == 1, true, false)}” style=”background:yellow;” value=”{!acc.Rating}”/>
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:page>

In case of apex:repeat, we can just write as below

<apex:variable var=”i” value=”{!0}”/>
<apex:repeat>
      <apex:variable var=”i” value=”{!i+1}”/>
</apex:repeat>