Surely, the question is about what you need to prove to yourself you are cut out for trading, and whether your methodology is viable.
If you need to take 100 trades to prove your method as viable, then that is your metric.
If only 50, then that is.
If you can trade for appropriate number of trades, and get some return, and also, if you can stick to your rules for that period, then you're good to properly fund an account.
If your set of trades doesnt give a good return, or you cant stick to your rules, then you can decide how much 50 or 100 trades are worth to you, in terms of initial test capital to lose.
If all you have is £100, then its £1 a trade, or £2 a trade if 50 trades gives you some proof of concept and proof of your ability to stay the course.
Get over those hurdles, then you can decide to properly fund yourself.
Instead of "losing money", think of it as R&D investment.