There is none. Tibia.com itself says you can stack them as high as you want, so you could just get 8 voucher's and use them all at once f.e.
I recommend to not use more than 4 at a time btw, if you are a mage, take some hours of those 4 vouchers for mlvl, mate of mine did on double voucher only in 1 night 103-106. I know a guy that made 80-112 in 7 days on 4 vouchers