Pretty sure it depends on the slot but most fully wired pcie x16 slots will give 75w. Don’t take it as a given though so read what it says next to the slot on the motherboard.

Ripped from wiki

Power

x1 cards are limited to 0.5 A at +12 V (6 W) and 10 W combined.

x4 and wider cards are limited to 2.1 A at +12 V (25 W) and 25 W combined.

A full-sized x1 card may draw up to the 25 W limits after initialization and software configuration as a high-power device.

A full-sized x16 graphics card may draw up to 5.5 A at +12 V (66 W) and 75 W combined after initialization and software configuration as a high-power device.[22]: 38–39

The main 12 V power supply for the PCIe slot is pins B2, B3 (side B) and pins A2, A3 (side A). Power standby 3.3 V is pin B10 and A10. PCIe x1 cards can receive up to 25 W and x16 graphics cards can receive up to 75 W.

My advice would be to consider throttling the bandwidth on the guest network and also block ports and use a restricted dns server with that vlan.

You can’t vet everyone’s devices so you want to be proactive.