I had someone ask me that a few times. as most will say there is no answer for how many dives. I am experienced ow and aow poor rescue functional trimix deep diver ok dry suit. get the drift. one can only look at the end of hte day and evaluate how the dives went. make a list of beginners mistakes and when you dont do them or get n a situatoin where you do not rely on them then you are non longer a beginner. be critical of your self but now over expecting. If you see shooting MSB's as a avdd ow then use MSB shooting as a measure of being an beginner aow or not. all sounds prety easy but , and this is at risk, you now can not decide what is a beginners set of skills and what is not, which instructor thinks deco is a needed ow skill or not etc. Three are not a lot of skills in ow to master. that said there are 2 parts to that the number of skills and the mastering of them. for ow becasue of few skills ,,,, however, those few skills are vital to master. they are the most necessary life support skills there are. some of the most vital is being aware of gas supplies and depth . then there are those things that are casually mentioned that always bite you like you tank band being loose when wet and your tank sliding out. You will know in your gut when you are no longer a beginner. sont forget that just because your tank does not fall off youare OK . same gors for your buddy. If his falls off,,,, you failed as a buddy.
You can say that perhaps after 25 50 100 dives you have been in every situation you can think of. perhaps true if yo have dove in 10 25 different locations. then you fall apart in the open ocean. fair critical self evaluation of your self and the input of your buddies and others will provide you your answer.
Dont forget that once you moved out of the beginner status that you can easily go right back there from lack of skill use.