You may encounter the terms rembetiko, rembetika, rempetiko, rempetiko or even rebetiko and rebetika. All of them are correct. These variations come from the "latinification" of Greek letters combined with plural-singular forms.
Most often "mb" is simply pronounced as "b" in Greek.
The Greek word is ρεμπέτικο - rembetiko (singular) and ρεμπέτικα - rembetika (plural).