Tricky quiz question

I've googled for ages on this, but can't make a 'breakthrough'. Any hints or nudges would be really appreciated. Question: 'There were none in the first two series and series three and four only had one each. They then gradually increased with 2 in series five, three in series six and 4 in series seven. Series nine had the highest to date with 12. There was speculation that there would not be any in series eleven but actually there were 3. Totalling 37 thus far – what are they?'