You can have only one bounty at a time to one question.
You can offer as many bounties on a question as you want. However, only one bounty can be active on a question at a time. Moreover, any user may have at most 3 concurrent bounties at a time.
Note that if you offer several bounties on the same question, you will have to double the amount each time (or more). That is, if your first bounty was worth 50 reputation, your second bounty on the same question will have to be for at least 100, your third for at least 200 and so on. If you've already offered a bounty for more than 250, you can still offer more bounties for 500 (the maximum amount) as long as you like (or as long as you have the rep). This doubling applies only to bounties by the same user on the same question.
For More Info: How does the bounty system work?