Some previous posts bring up some great points.
If you're going to bring more than one ball, each additional ball should provide a substantially different reaction, otherwise, it is no different than bringing one ball. We see too many bowlers adjust surface or layout to get that same look/reaction from all of their equipment because they can't stand leaving a ball unused until they find a condition that calls for it.
The differences should be easily noticeable. If you have 50 balls and each ball's reaction is just ever-so-slightly different from the other, you'll likely never get enough practice with all of them to truly understand and be able to use their nuances. That's when confusion can set as to what equipment to use in a situation when you need to make decisive moves.
The more different patterns you face, the more equipment can help you. If you're bowling on the same few (1-4?) patterns all the time, you shouldn't need much equipment to conquer them. If you're a tournament bowler that travels from town to town, state to state, country to country, it is likely that you'll see a wider range of conditions that will require a wider range of equipment. Again, the range of the equipment must be manageable or it is useless.
When I'm leaving the state or country to bowl, unless I have some insight as to what the conditions will be like, either from inside knowledge or experience, I favor the 6-ball bag as follows (You can read this very quickly as: Heavy early and late, medium early and late, light early and late)
1. Medium-Heavy to Heavy, early break, arc, dull solid
2. Medium-Heavy to Meavy, later break, hard arc
3. Medium, earlier break, arc, dull
4. Medium, later break, hard arc to snap, polished
5. Dry-Light, earlier break, arc
6. Dry-light, length, hard arc, polished pearl
For league nights and local bowling, two balls will typically be all you need, and will likely be two balls not on the above list. Indeed, many top amateur and professional bowlers have a completely different bag for league nights versus tournaments.
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Kill the back row (or maybe this should read "make your spares, dummy")
