Concelebration can be something of a mystery to many lay Catholics. In the parish, we most often see it when the bishop is on the scene, or when a special priest’s event, like an anniversary, retirement, or special visit takes place. Not too often is concelebration observed for a liturgical reason–at least not in parishes.
47. Concelebration of the eucharist aptly expresses the unity of the sacrifice and the priesthood; whenever the faithful take an active part, the unity of the people of God stands out in a special way, [See Sacrosanctum Concilium art. 57. Sacred Congregation of Rites, Decr. generale Ecclesiae semper, 7 March 1965.] particularly if the bishop presides. [See Sacrosanctum Concilium art. 41; Lumen gentium no. 28; Presbyterorum ordinis no. 7.]
Concelebration also symbolizes and strengthens the fraternal bond between priests, because “by virtue of the ordination to the priesthood that they share all are linked together in a close bond of brotherhood.” [Lumen gentium no. 28; Acta Apostolicae Sedis 57 (1965) 35; ConstDecrDecl 148; see also Presbyterorum ordinis no. 8.]
Unless the needs of the faithful (which always must be regarded with a deep pastoral concern) rule it out, then, and without prejudice to the option of every priest to celebrate Mass individually, this excellent way for priests to celebrate Mass is preferable in the case of communities of priests, their periodic meetings, or in other similar circumstances. Those who live in community or serve the same church should gladly welcome visiting priests to concelebrate with them.
I’m not sure what would constitute an example of the “needs of the faithful” ruling out concelebration–perhaps a particularly sensitive community of women.
The authorized superiors should therefore facilitate and encourage concelebration whenever pastoral needs or another reasonable cause does not demand otherwise.
The faculty to concelebrate also applies to the principal Masses in churches and public and semipublic oratories of seminaries, colleges, and ecclesiastical institutions, as well as in those of religious orders and societies of common life without vows. Where there are a great many priests, the authorized superior can allow several concelebrations to take place on the same day, but at different times or in different places of worship.
48. If a large host is baked for concelebration, as permitted in the Rite of Concelebration no. 17, care must be taken that, in keeping with traditional usage, it is of a form and appearance worthy of the eucharistic mystery.
Any comment on the “large host,” or anything else?
Catholic Sensibility 
Needs should not be confused with wants or preferences.
I can think of some situations that might reflect needs that should be deferred to:
1. Where concelebrants have acted in ways that brought scandal to congregants.
2. Where concelebrates would outnumber the people in the pews – that creates a weird imbalance.
3. WHere the concelebrants might not be familiar with the vernacular of most of the congregants.
These are unusual situations, and I am sure others can imagine some more.
It should be noted that the most current GIRM sets a norm that visiting priests (having a valid celebret, I assume) will vest and concelebrate at Mass.