How is RPC processed in the target ?

I have been using RPCs to process my players decision, so basically during an auction in the game, players will try to bid the price, which causes RPCs to send over the network to the Master Client ( I understand the Master Client can execute locally and resulting a faster bidding action ).

My question is, if multiple RPCs are called to the Master Client in a very close timing, are they process in a single thread manner ? or they are processed parallel ?

Though i saw threads in unity specifying unity is single threaded, i am not sure how PUN is design and embedded