Teşekkürler and Teşekkür ederim are same for meaning. Teşekkürler is thanks .... noun Teşekkür Teşekkür ederim is I thank you ....full sentence "verb:Teşekkür etmek"

"Teşekkürler" is just like Thanks, plural noun singular is Teşekkür also an expression of gratitude without the subject and object, usage is same as thanks depends on the context who says it who...where..."Teşekkür ederim" is where thanking is verb like "I thank" object is unclear but has the subject `I` embedded in the affix "eder(im)"... Just like in English, object is usually omitted in the dialogue or context where it's clear to whom it's directed at.