TurboTax Online can handle up to 1,500 transactions per brokerage account.
However, online performance may deteriorate as you get close to this limit.
If you have more than 1,500 stock transactions with a total sales volume of less than ten million dollars, you can switch to the TurboTax CD/Download software (which handles around 2,000 transactions per brokerage) or enter the transactions on Form 8949 using summary data from your broker statements.