There isn't a limit for how much you can declare for each charity but there are limits on how much you can deduct in total. In general, there are two limits on how much you can deduct, and they are extremely high. The majority of donations do not go over these limits.
One limit is 50% of your adjusted gross income if you gave to groups like Goodwill, churches and schools. The second limit is 30% of your adjusted gross income if you gave to groups like veterans groups and fraternal orders.
TurboTax will do the math as you work on your return.
To calculate how much each of your donations are worth, you would have to estimate its worth for its fair market value. This value is the amount someone would willingly pay for the item. You can estimate it by determining what you'd pay for the item at a thrift store. Once you are in > Deductions & Credits > Charitable Donations, in the "How Do You Want to Value Your Donations?" you will have an option to "Guide Me" (best for donations under $500) or "I'll Value Them" (best for donations over $500) to guide you through entering donations.