That man in question is John de Rither, who testified about his military service with the Scrope family in a court case over two knights (Sir Richard Scrope and Sir Richard Grosvenor) who were disputing over who had the right to wear a particular heraldric device. de Rither, as a long-serving man-at-arms associated with the Scropes, testified that he had fought since the age of 19, and listed his various forms of service, which included only 3 field battles but 8 sieges (and one naval engagement, at Sluys). Pitched battles, where two armies of thousands of men each fought in the open (as opposed to a siege), were comparatively rare, and often would be the culmination of a campaign. They were risky to engage in, and medieval commanders often sought to avoid them unless they felt they had an advantage or had no choice but to commit to a battle. Many campaigns would have ended without any sort of pitched engagement; de Rither's first military campaign, at the age of 19, was the inconclusive expedition that ended near Buironfosse in 1339, where French and English armies arrayed for battle but did not end up committing to a battle. However, medieval soldiers fought quite frequently even if pitched battles were rare.

de Rither fought in a number of campaigns and sieges, probably participating in dozens of ambushes, raids, skirmishes, and other small actions, many of which would have never been recorded. Most medieval sources are very elite-focused and naturally tend to focus on the largest and most important events which mattered most to the aristocracy. A major battle like Poitiers where a king was captured receives vastly more attention in medieval sources than the lives and experiences of the rank and file soldiery. Despite his fascinating experiences (including an expedition to Lithuania!), de Rither himself only becomes recorded in a document that survived to the present when he is testifying in court as a witness for Sir Richard Scrope's legal proceedings.

As for your other questions, the difference between a pitched battle and a skirmish is, at least for me, a difference in scale. There's not a hard and fast line, but an engagement like Crecy, where over ten thousand men on each side are arrayed against each other to fight in the field, is pretty distinct from a raid or ambush where a few dozen soldiers on each side are coming to blows. However, these kinds of smaller fights and actions are rarely recorded, making it difficult to determine exactly how many might have happened over a campaign or an individual soldier might have participated in. The careers of soldiers ranged very widely, from the veteran retainers like de Rither, who followed the Scrope family into battle everywhere from Spain to Prussia, to people like Geoffrey Chaucer (also testifying in that same case), who was an esquire of similar rank to de Rither, but only served in a couple of military campaigns and never fought in a pitched battle. For another, more quantitative example, Andrew Ayton examined the retinue that the Earl of Warwick took with him to Crecy in 1346. At least 65% of those soldiers had served with the Earl on at least one previous campaign. Warriors like de Rither were critical to the relatively small, increasingly professionalized contract armies of England in the Hundred Year War.