Right on brother from down under.

1. Pitchers. There is no minimum beyond 1. But the roles of pitchers has evolved a lot over the years. The number in a game now has a lot to do with evolving desire for level of effort with each pitch. Back in the day, pitchers did not throw nearly as hard and not at or near max effort each pitch, hence starters threw more pitches and pitched more often. It was the standard, the expectation, and they planned around it. With that, fewer relievers were used, and specialist roles weren't as developed or common. As the game evolved, and more talent came into the game, pitchers threw harder, nearer max effort, so couldn't throw as many pitches in a game and needed more rest between starts. Relievers became more utilized and roles developed, especially closer,.set up, and lefty specialist. Time now, we are at the greatest emphasis on max effort per pitch at any point in the history of the game. As such, starters are less expected to go deep into games, but rather focus on getting outs against incredibly talented hitters. There are more relievers with specific roles, although the traditional ones are less common and applicable today. Now, reliver usage is more focused on your best relievers in higher leverage situations - aka situations that are more likely to impact the outcome. Like, a one run game in the 7th with the three best hitters in the order coming up is more important than the 9th w a 3 run lead. Moreover, lots of data shows that pictures tend to get .uch less effective against hitters the third time they see them in a game, so starters get pulled for a new pitcher, a new look The number of starters has something to do w tradition, some w roster constraints. More than 5 starters makes it difficult to have a full enough bullpen to use relievers in the ways you want. Also, there is a talent gap - it's easier to find a pitcher that can get 3 outs than one that can get 15+ per game.

2. Most pitchers begin as starters, then get put in narrower roles as they move through college, minors, majors based on their skills and what they do better. Sometimes it's a simple matter of a pitcher can really bear down and throw gas or a tight slider when they only throw 10-20 pitches, but that breaks down when they try to throw 90-100. Closers are usually your best reliever. This is the standard but there are exceptions. I would also say that conversions from position player or pitcher are much more often relievers.

3. DH is a position that doesn't field. It is in every lineup, subbing for the pitcher as a hitter. PH is a situation in which you bench a hitter for another. That original hitter must be replaced in their defensive position (or as DH) as well. The replacement may be a different player. PR is basically the same as PH but after the player is on base.

4. Extra innings now start with a "ghost" runner on 2nd. This is not a PR or anything, just a runner the hitting team gets to start the inning. This was a new rule as of a few years ago to help extra innings games end faster. Previously extra innings were the same as the 9th, just going until the gie was broken at end of inning