THE "TWOFER" BOOM continues unabated. As one record company after another suddenly realizes that its archives are full of jazz recordings that today's audiences are likely to find fresh and exciting, the reissue field grows more and more crowded. There are now more than a half a dozen major labels with jazz reissue programs, all of them based on the "twofer" principle - two records for the price of one.

The latest entrant is Mercury Records, which recently began its "EmArchy Jazz Series" with re-releases of material by Clifford Brown, Dizzy Gillespie, Cannonball Adderley, Oscar Peterson and other jazz luminaries. The bebopping Brown sets are perhaps the strongest of the recent jazz reissues, but other packages cover everything from the pre-swing sound to the roots of the "free jazz" movement of the '60s:

CLIFFORD BROWN: "The" QUintet, Vol. 1 (Mercury EMS-2-403). One of the great talents of the late bebop era, trumpeter Brown died in an automobile accident in 1956. Only 25 at the time, he was already co-leading, with drummer Max Roach, a quintet so remarkable that it may actually merit the extravagant title bestowed upon it here. The group's entire recorded output has been collected in two marvellous volumes, and it shows Brown to have been an extraordinarily mature musician. His playing was a model of clarity, both in terms of the solos and melodic lines he devised and the clear, ringing tone of his instrument; "Daa-HOUd" and "Joy Spring" indicate that he also had considerable skill as a composer. Roach is strong throughout, too, and on the last half of Vol. 2. (Mercury SRM-2-407), there's an added treat: tenor titan-to-be Sonny Rollins replaces the capable Harold Land on saxophone.

FLETCHER HENDERSON: Developing an American Orchestra, 1923-1937 (The Smithsonian Collection R006). Although it never achieved the vast commercial success of some of its imitators, the Fletcher henderson Orchestra quite literally wrote the book on swing. Henderson sold some of his best arrangements to Benny Goodman, for example, and through his band passed some of the most brilliant soloists and arrangers in jazz history: Louis Armstrong, Coleman Hawkins, Benny Carter, Don Redman, Ben Webster. The evolution of this remarkable unit can probably best be determined by comparing "The Gouge of Armour Avenue," a 1927 number featuring tuba and banjo in the rhythm section, with the polished 1936 hit "Christopher Columbus;" but the differences between the 1928 and 1933 versions of "King Porter Stomp" point to considerable growth in an even shorter period. Also worth noting is "Hot and Anxious," a 1931 number that is a virtual dictionary of swing riffs.

LOUIS ARMSTRONG: Young Louis Armstraong, 1932-1933 (RCA Bluebird AXM2-15519). Once you get past the minstrel vocals and overdone sentiments of many of the tunes here, the real fun begins. It's largely a one-man show that Armstrong puts on, for though he is accompanied on numbers as different as "I've Got the World on a String" and "Basin Street Blues" by a 10-piece orchestra that inclused a young Teddy Wilson on piano, he takes most of the solos himself. And what glorious solos they are: the mutted introduction to "I Hate to Leave You Now," the heated middle section of "Mahogany Hall Stomp" and the soaring conclusion to "Sitting' in the Dark" are all prime examples of Satchmo's melodic creativity. A version of St. Louis Blues" with strong tango overtones shows his rhythmic interventiveness.

Jazz at the Philharmonic: Bird and Pres - The '46 Concerts (verve VE-2-2518). Since improvisation is the heart of jazz, there's nothing quite likealive album to capture the spirit of the music. A good example from these performances recorded in L.A. over 30 years ago is a 12-minute version of "I Got Rhythm" in which saxophonists Coleman Hawkins, Charlie Parker and Lester Young solo in dizzying succession. On the almost equally overwhelming "Sweet Georgia Brown," Hawkins drops out of the front line but is replaced by a bubbly Dizzy Gillespie. Most of the numbers here are long and leisurely - Gershwin's "The Man I Love" comes in at over 15 minutes - but the most celebrated bit from these famous concerts is one of the shortest: Charlie Parker's inspired alto sax foray on "Lady Be Good."

ERIC DOLPHY: Status (Milestone M-24070). Thirteen years after his death, Eric Dolphy is remembered mostly for a brief association with John Coltrane and his influence on reed man Anthony Braxton. That's hardly fair> for numbers such as the fascinating unaccompanied bass clarinet rendition of Billie Holiday's "God Bless the Child" included here show Dolphy to be an innovator in his own right. His fondness for instruments neglected by other players gave his music unusual colorations, and he had a striking harmonic sense - seen to best advantage here on a live version of Mal Waldron's "Status Seeking." Yet he could also swing - just listen to his flute on "April Fool."

FATS NAVARRO: Featured with the Tadd Dameron Band (Milestone M-47041). The sound is a little grainy, but the energy and joy of Fats Navarro's trumpet ring through loud and clear on this live date, recorded in New York in 1948. That, of course, was one of the peak years of the be bop era, and Navarro was one of the movement's leading lights. "Eb-pob" is a puckish excursion in which he uses mute and quotes from "Somebody Loves Me," but elsewhere he favors straight, brightly up-tempo numbers. Especially enjoyable are two versions of "Good Bait," one of the pianist Dameron's fienst riff tunes, and a surging, fast-paced reading of the Charlie Parker bop standard "Anthropology."

COUNT BASIE: Sixteen Men Swinging (Verve VE-2-2517). This is not the Basie Orchestra of the swing era, but a later unit, formed in the early '50s. Neal Hefti was writing and arranging for Basie then, but the real stars of the band were the "Two Franks:" saxophonists Foster and Wess. Though they are not especially impressive on set pieces such as "Down for the Count," they sound bold and bluesy on Duke Ellington's "Perdido" and the album's title tune. Basie and his piano stay mostly in the background, as is his habit, but on the Hefti-written "Softly with Feeling," the Count switches to organ and moves up front.