Yes, there are limits for topping up a phone using MyBambu. Here are the limits:

Daily volume limit: You can perform up to 5 reloads per day.
Weekly volume limit: You can perform up to 10 reloads per week.
Monthly volume limit: You can perform up to 25 reloads per month.
In addition to volume limits, there are also value limits for each recipient number:

Daily value limit: You can send up to $200 to a recipient number in a day.
Weekly value limit: You can send up to $400 to a recipient number in a week.
Monthly value limit: You can send up to $800 to a recipient number in a month.