Can i leave my iPad plugged in all the time ?

It is completely safe to leave your iPad connected to its Power Adapter for extended periods.

Whilst connected to the Power Adapter, the battery will charge - and the iPad will draw power combined from both battery and Power Adapter. In practice, if the Power Adapter is sufficiently high wattage, all power to run the iPad will be accommodated by the Power Adapter.

Depending upon your model of iPad, it will likely have been supplied with a 10W, 12W, 18W or 20W Power Adapter. You should note that most iPhone Power Adapters are only rated at 5W - and are insufficient to charge the majority of iPad models.

Essentially yes.

It is not recommended to use a Power Adapter rated at less that 10/12W with iPad. There is no guarantee that a 5W adapter will provide any useful contribution to power.

Your iPad8 supports USB-PD Fast Charging - and will optimally charge when using the supplied 18W (or greater) Power Adapter. Using a lower wattage charger with iPad8 provides no benefit.