여분필드를 사용해서

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<tr><td colspan=2 height=1 bgcolor=#F3F3F3></td></tr>
<tr class='ht'><td style='padding-left:20px; height:30px;'><font color=green><b>·</font></b>보험사 선택<font color=red>*</font></td>
<td><input name=ext10_12 type="checkbox" value="AIG생명" <? if ($ext10_12 == 'AIG생명')  echo'checked';?>>AIG생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_13 type="checkbox" value="동양생명" <? if ($ext10_13 == '동양생명')  echo'checked';?>>동양생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_14 type="checkbox" value="교보생명" <? if ($ext10_14 == '교보생명')  echo'checked';?>>교보생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_15 type="checkbox" value="뉴욕생명" <? if ($ext10_15 == '뉴욕생명')  echo'checked';?>>뉴욕생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_16 type="checkbox" value="대한생명" <? if ($ext10_16 == '대한생명')  echo'checked';?>>대한생명<br>
<input name=ext10_17 type="checkbox" value="동부생명" <? if ($ext10_17 == '동부생명')  echo'checked';?>>동부생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_18 type="checkbox" value="ING생명" <? if ($ext10_18 == 'ING생명')  echo'checked';?>>ING생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_19 type="checkbox" value="미래에셋생명" <? if ($ext10_19 == '미래에셋생명')  echo'checked';?>>미래에셋생명&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_20 type="checkbox" value="삼성생명" <? if ($ext10_20 == '삼성생명')  echo'checked';?>>삼성생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_21 type="checkbox" value="신한생명" <? if ($ext10_21 == '신한생명')  echo'checked';?>>신한생명<br>
<input name=ext10_22 type="checkbox" value="알리안츠생명" <? if ($ext10_22 == '알리안츠생명')  echo'checked';?>>알리안츠생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<input name=ext10_23 type="checkbox" value="푸르덴셜생명" <? if ($ext10_23 == '푸르덴셜생명')  echo'checked';?>>푸르덴셜생명&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</td></tr>
<tr class='ht'></tr>
<tr><td colspan=2 height=1 bgcolor=#F3F3F3></td></tr>

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이렇게 만들었습니다.

보험사 선택하는것인데..

 

체크박스를 4개까지만 선택할 수 있도록 하고싶은데..

그럴려면 어떻게 해야하나요?