Depends on what you want to define as 'successful'. For science, it's usually a p = 0.05 (lower the better).

To determine that, use something called a

* binomial table. *Or you use an online calculator:

https://stattrek.com/online-calculator/binomial.aspx#:~:text=What is the cumulative binomial,example of a cumulative probability.

If you're curious, binomial table looks like this:

So, take for example you want 20 trials. Assuming probability of each guess is 50%, if you guessed 14 correctly, that's a p = 0.037, or 3.7% you randomly got that result.

Then you have something called effect size, which is a whole can of marbles. Basically, if you can 'pass' the 0.05 benchmark, you can then determine if it's a large effect or small. To make it very simple, basically the larger the pass percentage (taking into account your sample), the greater the effect.

Summary: doesn't really matter unless you're trying to publish your results.