I want to connect my iPad os16. 8th generation to an external Acer monitor that accepts hdmi cable. What adapter do I need

The iPad User Guide is a rich (and often overlooked) source of information - and provides details of how you can connect the various models of iPad to an external monitor:

Connect iPad to a display with a cable - Apple Support

If your iPad has a Lightning port, you will require an Apple Lightning to Digital AV Adapter along with a suitable HDMI cable - or a Lightning to VGA Adapter for older monitors that only have a corresponding analogue video input.

• Lightning Digital AV Adapter

https://store.apple.com/xc/product/MD826ZM/A

• Lightning to VGA Adapter

https://store.apple.com/xc/product/MD825ZM/A

If instead your iPad has a USB-C port, the equivalent Adapters for HDMI and VGA:

• USB-C Digital AV Multiport Adapter

https://store.apple.com/xc/product/MUF82ZM/A

• USB-C VGA Multiport Adapter

https://store.apple.com/xc/product/MJ1L2ZM/A

If you have a 2021/2022 model of iPad Pro, for compatible monitors that support DisplayPort over USB-C or Thunderbolt3, you can also make a direct connection using an appropriate cable.

See if this helps

Charge and connect with the USB-C port on your iPad - Apple Support