On page 288 of the 20th edition book, it states that to resist a blood bond temporarily, the thrall must make a willpower roll and "accumulate a number of successes equal to the number of times the thrall has partaken of the regnant’s blood". Does this number cap at three? Or, for instance, if a thrall takes one drink a month for a year straight, would they need twelve successes to resist?
The number caps at 9, not 3.
It is possible, though difficult, for a vampire to temporarily resist a blood bond. Doing so requires the player to make a Willpower roll (difficulty is typically 8, though this can be modified depending on the regnant’s treatment and the thrall’s Nature) and accumulate a number of successes equal to the number of times the thrall has partaken of the regnant’s blood, to a maximum of difficulty 9. The thrall must then spend a Willpower point. Upon doing so, the bond is negated for a variable amount of time: from one scene (if the thrall merely wishes to plot against the regnant, deliver confidential information to an enemy, etc.) to one turn (if the thrall wishes to attack the regnant physically). The thrall can continue to expend Willpower to extend the duration of “freedom,” but once she ceases doing so, the blood bond resumes at full force.
As per a somewhat-poorly-edited sentence in that paragraph, the number of successes required on the Willpower roll to resist is equal to the number of times the vampire has drunk the regnant's blood, to the limit of 9. (The use of the word "Difficulty" here is a typo, as the difficulty of the roll is set earlier in the paragraph at "typically 8.")
There's actually a clarification of this in the Ghouls and Revenants book starting at page 36: 'Blood Bond Systems' that makes it slightly easier for a thrall to resist.
The amount of blood needed to retain a thrall under a regnant’s power is small, especially if the ghoul is already Bound. To maintain the blood bond, a vampire only needs to spend one blood point a couple of times a year. It should be noted that this is different from the monthly feedings required to maintain the condition of a ghoul
So note that there is a distinction here between blood points used to maintain the blood bond and blood points used to keep the ghoul a ghoul.
The first way a blood bond can be broken is if the regnant stops maintaining it. Generally, a thrall who neither sees nor feeds from her regnant for a period of (12 minus her permanent Willpower) months will find that her bond has been reduced by one level.
This means that you need to feed the thrall every (12 - Willpower) months or the blood bond will degrade. The paragraph that answers the original question explicitly is this:
Secondly, the more a ghoul resists the influence of the blood bond, the easier it is for that character to break it. Many abused ghouls, provided their sanity remains intact, try more frequently than those who are not. To resist, a Willpower roll must be made. The difficulty is typically 8, and the player must complete a minimum of three successes, one for each drink that forged the bond. A greater number of successes may be required, however, equal to how many times the thrall has fed on a regnant’s blood in the past 12 months. It should be noted that this resistance is specifically related to the vitae that caused the blood bond and not the ghoul condition.
Taking both paragraphs into consideration we can see that a 6 Willpower ghoul who had the blood bond reinforced twice in the last year (every 12-6 months) would only need (3 initial + 2 reinforce =) 5 successes to temporarily break the bond.
There isn't anything in there about overfeeding the ghoul to strengthen the blood bond but if you allowed that then the cap of 9 from the main V20 rulebook would apply.