There are negative effects that come from the first use, the second use, the third use, the fourth use, etc.
It is all cumulative damage, a VERY little bit of it at a time, but it is still relevant.
I would imagine, that contrary to the tendency to predict, the first trip would be the most damaging, and each individual trip after that would do progressively less damage.
So basically, you are the person who needs to decide how much is too much for you.
At what point do the recreational/spiritual/emotional/educational/interpersonal/other benefits of doing lsd outweigh the physiological and psychological losses?
Each and every time you dose, you need to ask yourself, 'Is it worth it?'.
When the answer is no, then that is too much.
Personally-- I don't see what you have to *gain* after the 20th trip besides a few hours of entertainment, and the losses and risks and costs are as heavy as ever.
And by that point you should have had a chance to try all the doses you want to, to have done it with all the people you want to, to have done all the things while on it that you wanted to... I don't see any reason to exceed that.
Thats really a serious psychological cut-off in my mind between 'I am a normal person' and 'I am not a normal person'.
Good luck not destroying your emotional health. (Seriously).