Any question answered...

Before I open a new chains packaging I need to make sure it will wrap around a 50t chainring and 32t lowest rear sprocket, chainstay is 43.5cm (50/34 chain set and 12/32 cassette . How many links am I likely to need?

Use a quick link and set the chain length with the chain on 50/32 with the derailleur arm at about 45 degrees. If the derailleur doesn't have enough movement to tension the chain on 34/12 remove more links. Remember that you shouldn't be using the 50 with the 32, so it doesn't matter too much if this combination doesn't work.

PS: I presume you have got a long arm derailleur, I doubt that any short arm one will cover this range.

PPS: Are you climbing walls?