I don’t know about “as many as he could,” but I think priests are allowed to say an “extra” Mass on All Souls’ Day for the poor souls in Purgatory.
There’s a very good reason that there are limits to the number of times priests can say Mass in a day, and the number of times (two) that the faithful can receive Communion in a day. It is to prevent it from ever becoming routine or mechanical (perhaps a poor choice of words, but it’s all I could think of).
The following is an excerpt from a pastoral letter written by Bishop Joseph Martino to all the priests in the Diocese of Scranton:
It is the Church’s ancient discipline and practice that, except for very special occasions like Christmas and All Souls Day, her priests celebrate only one Mass a day. So august is God’s gift of the Eucharist, so important is the spiritual preparation for it, so careful and attentive must its celebration be, and so essential the thanksgiving to be made afterward as priests carry forth its grace to the rest of their ministry, that the multiplication of this central act in a priest’s daily life runs the risk of diminishing the value he places on it. Such a danger imperils the whole community of faith along with its priest. The law, therefore, is not an arbitrary one. It provides an essential means of fostering the holiness of the Church’s faithful.
I imagine that the lay faithful would run the same risk if they received Communion too many times each day. The rest of the letter can be read at http://www.dioceseofscranton.org/Bishop’s%20Pastoral%20Letters/MultipleMasses.asp