In answer to your question" how many do I need", Just one, for the bellhousing drain hole, a Tdi will need another for the timing cover drain.
Fit them before you go in water, take them out at the end of the day or soon after, water gets in through other gaps and must be let out.
Axles, gear and transfer boxes can, and should be fitted with remote breathers if wading, as a hot axle, submerged when wading can suck in a lot of water, ( up to 2 liters).
Search eBay for ukmudmaster for kits and plugs, or PM me for price and or (free) advice.
Dave