Yes you can use the whole $50 for plugin transfers, it should be great to activate several.
Don't take my word for it, i don't have a correct current calculation but i think:
As the minimum spend for this voucher is $74, your cart total needs to be at least $74.
A plugin transfer is $20. 2 x $20 =$40, 3 x $20 = $60, 4 x $20 = $80.
So it can be used on activating 4 plugins for sure (and will save you money), BUT as 'dynamic discounts' apply, possibly you can use it on 3 transfers already OR, on 3 plugin transfers and the cheapest new plugin you can find (many ppl threw in 'BX Tuners for $5 to get over a threshold for example).
Here's more info on dynamic discounts:
https://www.plugin-alliance.com/en/news ... ounts.html
PM me if you're still interested
EDIT: Voucher sold