The number of farms in Maine has grown in the latest federal census, which shows a national decline. The US Department of Agriculture 2012 census found that there were 8,174 Maine farms in 2012, an increase of a half percentage point from five years ago. But the 10-year trend is more dramatic, with an increase of 13.6 percent. The preliminary data released Thursday indicated that the market value of Maine’s crops and livestock grew 24 percent. The USDA will release the full report in the spring. The data, collected every five years, shows Maine leads New England for its total number of farms.