Missing New button on One2Many with more than 2 records

From https://discuss.tryton.org/t/one2many-new-button-disappear-after-adding-records/6089

The field size is wrongly computed as it deduct an Array from size for which JavaScript as weird behavior:

>> -1 - []
-1
>> -1 - [1]
-2
>> -1 - [1, 2]
NaN