How risky is wanking?

Having a wank by yourself is about as risk free as sex can get. Wanking with someone else isn't much riskier. There are no records of anyone catching HIV from wanking.

STIs can be transmitted if you rub someone else's cum on the head of your cock. In theory HIV in cum can enter your bloodstream through your foreskin, the head of your cock or your urethra (the tube you piss through). If you have been wanking for a while and your cock is tender and a little bit sore, this could make it easier for HIV to enter your bloodstream.

Another risk is if you or your partner gets a little over-eager and pulls too hard (or too often), which can damage your urethra and lead to NSU.