How often should I place my drop wires?

I do from 4 to 8 feet apart. If you have a lot of short pieces of track I would stick with 4 ft. Longer pieces like 30 or 40 inch pieces you can spread them out. 

Instead of feet of track per drop, I believe it is more judicious to consider how many track joints per drop, as it is the track joints that are usually the source of power distribution low voltage zones. I have been using the 37" gargraves flex track, and tried to have no more than three track joints between drops.

  We have a 32 x 14 foot module layout using MTH realtrax. We use only 2 drops per main line.

Track joints per drop imo too.

Z controller? A model Z transformer with 4 dials? That is a ZWs predecessor, nearly the same umph as a ZW , but the ZW has whistle control.  Or MTH Z-750 or Z-1000, etc.  (imo they "copied" the Z card in their power names; followed by random numbers rather than risk any brand confusion to new buyers   

Either way, unless you have some monster supply by a "custom house" you are looking good on busses and drop sizes.

After enough years you'll want drops to be every other or every third section

I think Barry recommended every 3rd or 4th and lighter gauge wire for most MTH command applications & new track, for best command signal.

Conventional running will love the heavy gauge buses though, and other folk do use heavy wire with MTH no problem. (lost signals may be the wire gauge.... it's in the winding style too. Similar to high freq. delivery to speakers can be muddied with overgauging. "May happen" applies, but "rare"does  too. I don't recall a post here that ever stated "I ran too fat a wire".

Age has a lot to do with track connection quality. Cleaning inside each rail, both ends, isn't easy. More drops means the need to do won't ever likely be there

How far away from the power source you deliver X amps will determine minimum gauge needed to stop voltage drops at that distance.  An AWG chart online is easy to find.   

200ft one way (distance)? or 200ft total (loop)?

(fighting the auto composer changing words to nonsense got knarly, I kinda gave up)

Conventional tubular track layout using lockons and 18 ga wire, every 6 to 8 feet works fine, running conventional prewar and postwar trains

and several clockwork, they don't seem to care

MED posted:

Instead of feet of track per drop, I believe it is more judicious to consider how many track joints per drop, as it is the track joints that are usually the source of power distribution low voltage zones. I have been using the 37" gargraves flex track, and tried to have no more than three track joints between drops.

Pretty much the same as MED.  I have a drop on every other 37"  section of GG track using 16 ga. stranded wire.

Putting drops in after the track is laid is hard. Putting them in while laying track is easy. You do not need to use all of them if you do not want to.

Don't forget that with both Fastrack and Realtrax every track switch provides an easy track power drop. 

With Fastrack just connect your power drop Ground wire to "AUX GRD" and your power drop hot wire to "TRACK JUMPER". With the jumper removed separate switch-motor power is connected to "AUX IN".

With Realtrax the right-most terminal (with red wires connected to it) is track power "hot" while the third-from-right terminal (with light blue wire connected) is for track power ground. The second-from-right terminal is for separate switch motor power (with the top-side jumper removed).

Place a drop every tenth section of track.  And if you plan to use DCS, isolate the blocks and use a star pattern for your drops.