It all depends on competition in the particular subreddit. For the biggest subreddits, you usually need just about 50-70 upvotes to get into the top 5 hot ones. If the audience likes the post, it will naturally climb even higher and stay there for a long time. If they don't like it, it will go down.
For small subreddits with less than 500K subscribers, sometimes 10-15 upvotes are enough.
You can buy in small quantities (10-20 upvotes) and monitor your current position. If your position remains low, click "repeat order" and specify how much more you want to add. This would save you from spending money on wasted votes and with time you’ll be able to know how many upvotes to order and in which subreddit.
Soon we will implement the system to be able to automatically stop upvoting when the desired position has been reached. You only need to specify what position you want and how much you're willing to spend.