the effective range of SPI clock setting on ARDUINO UNO REV3 board

slave board: ARDUINO UNO REV3
SPI API: beginTransaction(SPISettings(uint32_t clock, uint8_t bitOrder, uint8_t dataMode))
Who know the the effective range of clock setting?

ATmega328P.pdf has the following content:
• Bit 0 - SPI2X: Double SPI Speed Bit
When this bit is written logic one the SPI speed (SCK Frequency) will be doubled when the SPI
is in Master mode (see Table 18-5). This means that the minimum SCK period will be two CPU
clock periods. When the SPI is configured as Slave, the SPI is only guaranteed to work at fosc/4
or lower.
How many mHZ match "fosc/4"?

How many mHZ match "fosc/4"?

Assuming you don't actually mean mHz but MHz then the oscillator frequency on most AVR Arduino's is 16MHz so a quarter of that will be 4MHz.

The effective clock limit on a bus system depends on its slowest client, and on the quality of the cabling.

See the online reference manual for a list of available clock divider constants: