So lets say you purchased 100 shares of something for $10 each. Leaving out the subject of commission fees (which is a flat rate, usually around $10 depending on your broker). That means you spent $1000. Now lets say the price of the stock goes up to $12 per share. $12 x 100 shares = $1200, a gain of $200. However, that gain is an "unrealized" profit because you didn't actually sell anything yet. So lets say now you sold half your shares. You don't necessarily have to sell half, you can sell any number between 1 and all your shares (100 in this case), OR you don't have to sell any at all - you can keep them forever, the end of the day, the end of the week, next year, 20 years from now, til you die and pass it onto your kids, etc. Anyways, lets say you sell half at $12 a share. 50 shares x $12 = $600. Since you sold them, now those profits are called "realized". (which you can be taxed on unless you're trading in a tax shielded account like an IRA, but you can find out about that later).

So, 50 shares left, at $12, because you sold the other 50 for a profit of $2 each, or $100 total profit (realized). Now lets say that the stock goes down to $6. You are now at a (unrealized) loss of $4 per share (since you bought them at $10) or, $200 total (unrealized) loss (50 shares x $4 = $200). This is what people are talking about when they are at a loss. Now lets say the shares go back up to $8, and you decide to sell the rest of your 50 shares, at $8 each. You will get (50 shares x $8 = $400), a loss of $100 "realized".

You may be over-complicating things a bit, you can sell any percentage of your portfolio that you choose, when they are gone they are gone. The value of stocks go up and down daily, but it doesn't translate to a finantial gain or less until the moment you sell them.

Beat in mind that broker fees need to be considered as well, buying and selling too frequently can lose you money just from fees.

Yes you can sell as many as you want but keep in mind each sale has fees that eat into profits

I would think holding on to some of the shares to see if the price per stock continues to increase would be wise.

But this also confuses be because say I hold on to 50 of the shares and the price for stock dives that day. What happens with my 50 shares? why would ppl say that is a loss? if perhaps in 5 days price may go up again, can't I then sell those 50 shares? or once the trading day closes I forfit those 50 shares and no longer own them? Thank you for clarification.