If you have a 10k account - no matter what you buy or do, that wont affect the market in any way But how much money do you need to actually affect the market? To move the price of a stock you buy?
Depends what you're doing .... If you're buying AAPL stock, then yeah.... 10k is just noise. If you're buying some illiquid stock, then its a different story....
I've purchased small cap stocks in large enough quantity with a single buy order that the spread between the fills was noticeable. And it took a few minutes to fill. I've purchased SPY ETF in large quantities that didn't have any effect on the price history. If you are talking large cap liquid stocks, you would have to purchase a *VERY* large order (millions of $ worth?). If you are talking small cap illiquid stocks, 1 or 2 lots might shift it. Although to be honest, in this situation, I think it is less the fair market value being shifted but the market maker that is moving his prices around. Sometimes, if I'm bored, I'll put in buy orders on a wide spread (say 50c) illiquid stock that is about $0.03 above the highest bid, and then watch all the market makers move up to match me
Just enough to buy the last share currently available at the price at which you buy. $5 might be enough; $1,000,000 might not be enough.
If you trade options in the US equity market, sometimes a few contracts will move the market if the options are thinly traded. I often traded options on thinly traded market and it took only a few contracts to move the bid/ask. In those cases my counter parties were almost always the market makers. How did I know? I could tell from the bid ask spread and the calculated the IV of the bid/ask.
The rule of thumb is that anything over 20% of ADV should move the price. There are standard ways of estimating how much, that's what block desks do