Robert Bruce only had 1 son, David, who did not have any children despite several marriages. Although Robert has a very large number of descendants, they are all through his daughters. Therefore, any descendants that coincidentally happen to be called Bruce get that from a different paternal line.
Given the way that some of his eldest daughter's descendants behaved, a large proportion of the population of Scotland may be descended from him - in the female line.
Anyone can 'claim' descent from anyone, but the vast majority of such claims turn out to be wishful thinking based on having the same surname - even if it was an ancestor centuries ago who did the wishful thinking.
Many very longstanding claims of this nature have recently been proved false by DNA tests.