Try using a combination of two rules in your grxml file.
First rule to accept individual numbers:
<rule id="SmallNum" scope="public">
<one-of>
<item>One</item>
<item>Two</item>
<item>Three</item>
<item>Four</item>
<item>Five</item>
<item>Six</item>
<item>Seven</item>
<item>Eight</item>
<item>Nine</item>
<item>Zero</item>
</one-of>
</rule>
Second rule to build an account number from a specified number of numbers:
<rule id="AccountNumber" scope="public">
<item repeat="0-1">AccountNumber</item>
<item repeat="3">
<ruleref uri="#SmallNum"/>
</item>
</rule>