The #<number>, such as #0 and #1, is the location number and indicates which combination (Location) was used to assign the peripheral to the pin.

Many peripherals, such as ADC and I2C, use I/O pins.
However, since the number of pins is limited, a mechanism is provided that allows you to select pins to be used in peripherals from multiple candidates.
The figure below is an excerpt from the EFM32GG990 datasheet "Table 4.2. Alternate functionalityoverview". (The order has been edited for clarity)

USART0 uses a total of 4 pins, US0_CLK, US0_CS, US0_RX, and US0_TX. When used at Location 0, 4 pins of PE10, PE11, PE12, and PE13 are used as a set, and when used at Location 1, PE4 , PE5, PE6, and PE7 are used as a set.
US0_CLK cannot be used in different locations, such as Location 0, US0_CS in Location 1, and so on.

The figure below is an excerpt from the EFM32GG990 datasheet "Table 4.1. Device Pinout". (The order has been edited for clarity)

There is US0_xx #2 in the Communication column, which indicates Location 2 of USART0. Setting USART0 to Location 2 will occupy 4 pins, Pin# C10, C11, D10, D11.

Notation such as #1/2 is an abbreviation for #1 and #2.