If I am investing in individual stocks in my Roth IRA and decide to sell to lock-in profit, am I taxed on this, or is there any limit to the amount of times I do this?

Transactions within Roths and regular IRAs:

Regular and Roths: no tax on transactions, no specific reporting requirements, keep the records you need to know how you are doing (very important!), no restriction on number of trades or transactions or length of hold (long term vs short term is meaningless concept

Roths only: normally no federal income tax liability with eventual distributions unless Congress changes the rules (IRA, alt min tax, etc.)

IRA only: taxed as ordinary income when distributed later in life {note: you can not get favorable long term capital gains treatment}

The above generalizations do not address premature withdrawals or state/local taxation.

I would discourage very active trading in a Roth or regular IRA. Very few folks are successful at this, perhaps less than 20%. You don't need to be swinging for home runs to get an IRA account to grow to a substantial size. If you choose to actively trade, you may want to consider an internet based discount brokerage to reduce your transaction costs.