Since no novel is perfect everynovel there has one. But when is too much plotholes too much?
When do readers mind plotholes?
When it disrupts the reading. Some plot holes can be overseen, and others clearly offend the built logic ( I mean, the world-building, the characters' behavior, etc) and the reader's intelligence.
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When they pay attention or have standards.
when they don't notice it until they've read the book another time
My opinion is that every plothole has an explanation to it, you just have to find it.
Now, the amount of searching you have to do to find it is what really shows whether a plothole is bearable or not.
For example, say a newly introduced character has a change in the tone of their voice.
This could be considered a 'plothole' in a very liberal use of the term, because they would be saying something 'out of character', however at the same time, the viewers can't really say they know enough about that character for it to be out of character.
Say this happens for a well established character where they randomly do something out of character.
This could be passed off as a hidden side to the character.
Now, as far as actual plot, similar logic can be applied. But, when something just straight up contradicts itself, thats when you have an issue. When you say one thing at one point, and later on it changes and becomes something else. Etc. These are the real issues. Fix these.