How can I add to a number in project properties through context?

I believe you are reading it groovy and do an increment?

then as shown below

def projPropertyValue = context.testCase.testSuite.project.getPropertyValue('fpl_us48_single') as integer

Then you  will be able to do all math operations such add, subtract

While storing back to property, write it as String

context.testCase.testSuite.project.setPropertyValue('fpl_us48_single', projPropertyValue as String)