The below is my code. Question is why this is displaying 20 account records instead of all? 

<apex:page standardController="Account" recordSetVar="accounts">
   <apex:pageBlock title="Accounts">
       <apex:pageBlockTable value="{!accounts}" var="a" rows="1000">
       <apex:column headerValue="Name">
        <apex:outputLink value="/{!a.Id}"> {!a.name} </apex:outputLink>
        </apex:column>
       </apex:pageBlockTable>
   </apex:pageBlock>
</apex:page>