Relation between Mbits/s and MHz
(Updated after P__J__'s comment)

Yes, if you count within one byte (see explanation below).
No, if you count over multiple bytes, because of the gap between the bytes.

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(Note that instead of ASCII, any data can be sent).

As you can see the clock speed has an up/down pulse, and this corresponds to 1 bit. So 1 mbits/s equals the 1 MHz clock speed (or in other words, in 1 million clock pulses, 1 million bits can be sent/received).

This is not mandatory for all protocols, but for SPI it is.