The 1951-S Franklin Half Dollar is one of the more common dates and has one of the higher mintages for the series. Most Franklin Half Dollars have mintages under 10 million and any coin with a mintage higher than that is usually very common in most circulated grades.
The 1951-S seems to be common up to MS63 grades and can still be found in Uncirculated rolls in this condition and lower. In MS64 condition most examples have been pulled from rolls and are sold as singles or many times graded already. In MS65 condition they become a little scarcer especially with the Full Bell Lines designation. In MS66 FBL there are probably less than 1,000 coins that survive in this condition or higher. In MS67 FB it is really scarce with less than a dozen known and none being known in any higher condition.