So I was looking at alternate functions of the GPIO pins of the Raspberry Pi 3b+ and found that some GPIO pins do support txd and rxd alternate functions. However some are named txd0/rxd0 and some are named txd1/rxd1, and they appear multiple times in different GPIOs. Does anyone know what this means? Does this mean I only have 2 sets of txd/rxd pins?
The Pi has many hardware peripherals. The first of each type is labelled with a 0, if there are more than one of that type of hardware peripheral subsequent ones are named 1, 2, etc.
The hardware peripherals may be multiplexed onto more than one group of GPIO.
In your case you see TX 0 and TX 1 which indicates there are two hardware UARTs.
UART 0 may be multiplexed to GPIO 14/15 and GPIO 32/33.
UART 1 may also be multiplexed to GPIO 14/15 and GPIO 32/33.
The Answer by Joan is correct. But to clarify you can only use ONE serial interface on the GPIO.
On all models with on-board WiFi the primary serial interface is connected to Bluetooth. The secondary interface has a few limitations (although for most uses these are not an issue). See How do I make serial work on the Raspberry Pi3 , Pi3B+, PiZeroW