I rely on the Concealed Combat Commander IWB or OWB at 4:00 and 2 8's at 8:00. 25 rds of 45 acp in two quick mag changes works for me. The Cell Phone is at 9:00, and I have always practiced reloads with the cell phone on board to make sure I don't accidentally try stuffing it up the mag well. Re-loads are very fast. It is terrifying to imagine a situation requiring more than 3 shots or 3 seconds before or after movement or heading to cover. Unless the weak hand is controlling the shurefire flashlight, it should soon be holding a spare mag.
The role of the BUG seems to be defined rather individually, and most are trying to give it a great deal of importance. Absolutely nothing can replace the access, capacity with re-loads, speed, control, or power of the primary. The primary should be THE one with which to practice, to carry always, and to count on absolutely. Keep it simple, fast, and effective.
IMO the BUG ought to be as small, smooth, light, and thin as possible for deep concealment and stealth. It should be carried strong side when by itself, and weak side when complimentary to the primary. The NAA 380 Guardian and the Kel-Tec P3-AT come to mind. Revolvers are bulkier, snaggier, and more difficult to conceal, but some work well as BUGs. By its nature, deep concealment precludes the NY re-load due to the more difficult access, unless the primary ammo has been used. If the BUG's role is expanded, it must become larger, more powerful, and carried more accessibly, possibly interfering to some degree with the primary's fast presentation, and certainly very bulky and heavy for something so seldom needed.
IMO the possible needs for BUG are: strong side shot, damaged, or blocked; primary ammo exhausted; primary malfunction; or loss of retention of primary, in that order of probability. I carry one extra mag for the BUG in a custom sleeve on the pocket holster, but I doubt I could re-load it if the strong side is shot up or blocked, anyway.