No limit. The auction contract is profit maximizing and will spend the top bidders’ budgets as best possible to sell every unit for the most revenue possible. 

For example, if if bid $5000 total budget and you bid $2500 and we are the only two bidders, it’s most profitable to sell 3,333 to me for $1.50 and 1.1111 to you for $1.50

fyi, for the upcoming $moxie_ai auction...

TL;DR 

the higher your bid, the more likely you are to receive more $moxie_ai

Playing slots, gambling, betting on sports.