The Ramaphosa MP's (there are definitely many more than the 50 needed) will join the opposition in parliament to remove Zuma as president in early January. It will be a secret ballot (Mbete is now openly in the CR camp).
She as speaker will be appointed president when parliament fails to replace Zuma. Elections will then be held within a month.
The ANC will be spilt and in disarry, with the 2 factions competing against each other in the elections.
That's not exactly how an event like that would take place.
Firstly, the ANC would need to hold a vote of no confidence in Zuma, and force him to resign (impeachment might just break his patience). Then, with Zuma's resignation, the DP and the rest of his cabinet resigns and become ordinary MPs. Voting in parliament than takes place to elect a new President, and during this time Mbete is acting president.
The only point at which special elections would be held is if parliament cannot elect a replacement president and the situation draws on longer than 30 days.
Section 83.3:
An election to fill a vacancy in the office of President must be held at a time and on a date determined by the Chief Justice, but not more than 30 days after the vacancy occurs.
Section 50.1:
50. (1) The President must dissolve the National Assembly if—
(a) the Assembly has adopted a resolution to dissolve with a supporting vote of a
majority of its members; and
(b) three years have passed since the Assembly was elected.
(2) The Acting President must dissolve the National Assembly if—
(a) there is a vacancy in the office of President; and
(b) the Assembly fails to elect a new President within 30 days after the vacancy
occurred.
... difficult to pick which one is worse to be stuck with 
