By infallibility, the CC means that she is preserved from making errors when defining matters of faith and morals.
The voice of infallible authority can come to the Church in 3 ways:
- Through the Pope by an ex cathedra pronouncement.
- Through an ecumenical council (the bishops of the universal Church in union with the Pope.)
- Through the ordinary magisterium (ordinary exercise of the authority of the bishops of the universal Church in union with the Pope.)
A canonization would fall into the first category–an ex cathedra pronouncement of the Pope. Most Catholic theologians believe this to be the case; however, the Church has never defined it as such.
To show you what the issues are, this is from the Catholic Encyclopedia:
newadvent.org/cathen/02364b.htm
PAPAL INFALLIBILITY AND CANONIZATION
So, to answer your question of “Is the canonization process considered to be part of the infallibility of the church?”
Most theologians believe it is, but the Church has not defined it as such.
You asked " why do many apologists say that the pope has only made a few infallible decrees?"
Many of the teachings of the Church are infallible because they were defined by ecumenical councils or by the ordinary magisterium. But as far as ex cathedra pronouncements of the Pope, there are only two: the doctrines of the Immaculate Conception in 1854 and the Assumption of Mary in 1950.
The Church may decide in the future that a canonization does meet the criteria for an ex cathedra pronouncement. But at this time, it is still a matter of debate.