Wasim said the squad will be reduced to 16 players after a week.
“I have been consulting captain Babar Azam, head coach Misbah-ul-Haq and the head coaches of the provincial teams who are also national selectors before I finalise the squad,” he said.
The first Test will be played at the National Stadium in Karachi from January 26, followed by the second one at Rawalpindi (February 4-8) and three T20 Internationals in Lahore.
The former batsman, who has played 18 Tests, said the performance of the players during the recent tour of New Zealand and of those in domestic cricket would be taken into consideration in the selection of the squad.
He said importance will be given to those who had performed well in the just concluded Quaid-e-Azam Trophy first class tournament.
A reliable source said that some changes can be expected in the Pakistan Test squad with top domestic performers like spinners Zahid Mahmood and Nauman Ali, pacer Waqas Maqsood and batsmen like Saud Shakeel, Kamran Ghulam and Usman Salahudin are likely to be included in the initial 20-member squad.
“There is also a chance that seasoned batsman Asad Shafiq who was dropped for the New Zealand tour will be recalled for the South Africa series,” the source said.
The South African cricketers arrive on Saturday in Karachi for their first visit to Pakistan since 2007 and will spend four to five days in isolation at their hotel before they start training.
South Africa, led by Quinton de Kock, have announced a strong squad for the tour with fast bowler Kagiso Rabada making a return after injury issues.