To answer the question as asked you would need to provide a robust definition of front lines and king. You would also need to make it clear if you are asking how common was it for any given king to appear on the front line at least once or how often did the average battle feature a king on the front line.

Oh and you still won't get an answer because depending on how you define "king" you would need someone with at least a reasonable working knowledge of every culture in question (unless you have a very narrow definition of kings). There aren't that many people who know their way around Irish kings, Hawaiian Kings and Egyptian Pharaohs (are they kings or not?).

Still riffing in a question appears to be an accepted practice on askhistorians so lets look at English kings.

In general up until restoration in 1660 kings were expected to lead their own armies. Not that armies wouldn't be lead by other people but the loyalty of such armies would always be a bit suspect. How often such kings ended up on front lines is harder to say. Certainly by the civil war Charles I was actively prevented from getting engaged in personal combat.

So how many did end up on the front lines? One approach is to look at who died in battle. The most unambiguous case was Richard III who was actually in front of his main troop lines when killed at Bosworth. Harold Godwinson exact position at the time of his death is unclear although given that his position was over run he would have been on the front lines if he was alive at that point (which is unclear).

An alternative to being killed is being captured which Stephen managed Battle of Lincoln. Descriptions of the event make it pretty clear he was involved in hand to hand fighting.

Another options is being involved in a siege Edward I of England was besieged inside Conwy Castle in 1294. If he ever stepped onto the outer walls or towers (which given the castle's design is pretty likely) he would have been on the nominal front lines.

In general kings were expected to be able to fight at least while of a reasonable age but their actual appearance on the front lines would depend on tactical considerations and aren't always recorded.