I've just come across this exact same issue. There are absolutely no T&Cs to suggest there is a daily limit, but when I called up and spoke to AirBnB, I was advised that I have reached my daily. limit for redeeming gift cards! When I asked what that limit was, they told me they couldn't disclose what that daily limit is?! Surely legally they have to tell us this information because I wouldn't have purchased so many gift cards!
I have agreed with a villa that they will hold it for me until today, and now have £4k worth of vouchers sat there that I am unable to redeem and I won't be able to use against this booking!