Two parts
Two-member rock and pop bands are relatively rare, because it is easier to provide all of the musical elements which are part of the rock or pop sound (vocals, chords, bass lines, and percussion or drumming) with trios or quartets. Two-member rock and pop bands typically omit one of these musical elements. In many cases, two-member bands will omit a drummer, since guitars, bass guitars, and keyboards can all be used provide a rhythmic pulse.

When electronic sequencers became widely available in the 1980s, this made it easier for two-member bands to add in musical elements that the two band members were not able to perform. Sequencers allowed bands to pre-program some elements of their performance, such as an electronic drum part and a synth-bass line. Two-member pop music bands such as Soft Cell used pre-programmed sequencers.

Other pop bands from the 1980s which were ostensibly fronted by two performers, such as Wham and Tears for Fears, were not actually two-piece ensembles, because other instrumental musicians were used "behind the scenes" to fill out the sound. Two-piece bands in rock music are rare. In the 2000s, blues-influenced rock bands such as The White Stripes and The Black Keys are duos that use an electric guitarist and a drummer.

Three-member ensembles
The smallest ensemble that is commonly used in rock music is the trio format. In a hard rock or blues-rock band, or heavy metal rock group, a "power trio" format is often used, which consists of an electric guitar player, an electric bass guitar player and a drummer, and typically one or more of these musicians also sing. Several well-known power trios from the 1960s include the Jimi Hendrix Experience and Eric Clapton's "supergroup" Cream. Many four-piece rock groups from the 1960s and 1970s (such as The Who and Led Zeppelin) can be considered as instrumental power trios with the addition of a vocalist.

An alternative to the power trio are organ trios formed with an electric guitarist, a drummer and a keyboardist. Although organ trios are most commonly associated with 1950s and 1960s jazz organ trio groups such as those led by organist Jimmy Smith, there are also organ trios in rock-oriented styles, such as jazz-rock fusion and Grateful Dead-influenced jam bands. In organ trios, the keyboard player typically plays a Hammond organ or similar instrument, which permits the keyboard player to perform bass lines, chords, and lead lines. A variant of the organ trio are trios formed with an electric guitarist, a drummer and an electronic keyboardist (playing synthesizers) such as the progressive rock band Emerson, Lake & Palmer.

Four parts
The four-piece band is a common configuration in rock and pop music. Before the development of the electronic keyboard, the configuration was typically two guitarists (one lead guitarist and one rhythm guitarist), a bass player (typically the electric bass guitar) and a drummer (e.g., The Beatles). In some early rock bands, keyboardists were used, performing on piano (e.g., The Rolling Stones initially used Ian Stewart on piano), electric pianos (such as the Fender Rhodes), or organ (such as the Hammond organ). However, due to the difficulties of transporting pianos or 1950s and 1960s organs, smaller, lighter instruments such as guitars were typically preferred to keyboard instruments. In the 1980s, when lightweight, relatively inexpensive electronic keyboards became widely available, keyboards became common in four-piece bands. In the 1990s and 2000s, keyboards continue to be widely used in four-member rock ensembles.

Larger rock ensembles
Larger bands have long been a part of rock and pop music, in part due to the influence of the "singer accompanied with orchestra" model inherited from popular big-band jazz and swing and popularized by Frank Sinatra and Ella Fitzgerald. To create larger ensembles, rock bands often add additional guitarist (as in some line-ups of The Eagles); an additional keyboardist; additional percussionists (as in latin-rock bands or Slipknot); or second drummer (as in The Grateful Dead and The Allman Brothers Band). In other rock bands, particularly those influenced by other genres such as folk or country, the additional instrumentalists might include a mandolin player, fiddler, or a pedal steel guitarist. In some of these larger groups (such as The Band instrumentalists could play multiple instruments, which enabled the ensemble to create a wider variety of instrument combinations. More rarely, rock or pop groups will be accompanied in concerts by a full or partial symphony orchestra, where lush string-orchestra arrangements are used to flesh out the sound of slow ballads. -Wikipedia.org