How do I count the number of days I spent in a state? I was on an F-1 OPT in 2018.

California doesn't conform to the Federal law on residency issues or days exempt due to an F1 visa..  You need to input the number of days you were in CA.

Thank you for that! So am I considered a non-resident if I stayed in the US for more than 183 days on an F1 OPT visa?

Yes, for federal tax purposes.  If you have been in the US on an F1 visa for less than 5 years or part years you are a nonresident.  For those 5 years you do not count your days of presence for Federal tax purposes.  You should be filing a US nonresident tax return this year. TurboTax does not handle nonresident returns, only full year resident returns.  
However we have a partnership with Sprintax offering a nonresident tax filing solution to our TurboTax customers.
Visit the TurboTax/Sprintax site for more info or to get started.

Just to summarize your advice: I am a non-resident in the fed's eyes but a resident in California's eyes. I did leave California at the end of August. Does that mean I became a state non-resident in 2018?

Also, should I file both my state and federal taxes on Sprintax?

Sorry for asking so many questions!

I don't know whether you will be considered a California resident but you do not need to determine that.  The Sprintax software will determine that.  Yes, you need to file both federal and state return with Sprintax.  Happy to help!