Everything's a tradeoff.
Designing and building small, low-thrust engines is generally easier than designing and building large, high-thrust engines, but the larger engines may be more efficient.
The more engines you have, the more likely one or more are to fail in any given launch.
Once you have more than 5 or so, you may be able to continue on if one fails (depending on which engine, and at what point in the ascent it happens).
The more engines you have, the harder it is to predict how they may interact; this is likely part of what contributed to the failure of the N1.
Given that N1 never flew successfully, and Soyuz is really a 4 booster + 1 sustainer first stage, the "Russians use more engines" assertion is misleading. Energia was 4+4, but flew only twice; Zenit uses a single 4-chamber engine; Proton uses 6 single-chamber engines.
Almost every engine and launcher in production has a complex development history; sometimes your requirements change and you wind up building a much larger launcher than you had expected to.
Engine development generally takes longer than the rest of launcher development (tankage, avionics, airframe, etc.), so you often don't have time to develop the optimal engine for a new launcher. During the height of the Cold War/Space Race, if your government made a request for proposals for a new launcher, you'd look around at the engines you had already developed or had experience with, divide the needed thrust at launch by the thrust of one of those engines, and that's how many engines you'd design for.
SpaceX wanted to use a common engine design for Falcon 1, Falcon 5 and Falcon 9, and use the same engine in the second stages of the 5 and 9. The upper stage naturally needs less thrust, so they designed the Merlin to drive a single-engine second stage, and payload goals set the number of engines in the first stage accordingly.
The Saturn program was originally going to be a larger family of launchers. The Saturn C-3 was to use 2 of the enormous F-1 engines, the C-4 would use 4, and the C-5 became the Saturn V with 5 of them.