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window.onload = function(){
alert( document.getElementsByName('cat_select[]').length );
};
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<input type="checkbox" name="cat_select[]" id="model1" value="158" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model2" value="300" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model3" value="500" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model4" value="800" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model5" value="70" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model6" value="100" onClick="checkBlocking()"/><br>
<input type="checkbox" name="cat_select[]" id="model7" value="300" onClick="checkBlocking()"/>