私はそれぐらいしか思いつかない's meaning

 「私はそれぐらいしか思いつかない。」I can only think of those things.

 「私はこれぐらいしか思いつかない。」I can only think of these things.

This sentence is used very often. 

If you don't know the answer when the teacher asks you a question, you can use this sentence to show that you can't think of it.