The intent of restricting questions to fewer claims is to limit the effort needed to give a complete answer, as the Stack Exchange system has a single accept and requires one answer to be the better one to work fully.
As long as the effort required is reasonably limited, I personally see no issue in having more than one claim. The more closely related the claims are, the more numerous they can be, in a single question.
In fact, multiple claims should be grouped together in some cases in order to limit effort! For example, a single claim can have multiple slightly different versions and I would expect that all versions are grouped in a single question.
For these reasons I'm weary of specifying an exact number. In cases like the one you present, probably choosing one or two representative claims is best, for example asking whether immigrants in UK contribute more than they take seems to be a feasible alternative.