16Kb, and it was launched early in '83
The Spectrum was actually launched in Q1'83, and so this thread should really belong to the previous, wonderful gallery, together with the ZX-81.
And of course it was 16Kb in the base system. Although some of us were lucky enough to buy the one with 48Kb RAM.
And yes, we keep wondering how did you manage to code anything under 16Kb of RAM... until we remember that their whole OS was running within 16Kb of ROM.