Yes, you will likely be eligible for JEE Main 2025 and the subsequent counselling process even if you are giving an improvement exam in only one subject this year (2024).

You should have passed the Class 12 (or equivalent) exam in 2023 or 2024, or be appearing for it in 2025.
There is no minimum marks requirement for appearing in JEE Main 2025. However, securing admission into NITs, IIITs, and other CFTIs through JEE Main counselling typically requires a good score depending on the competition and your category.

Most engineering colleges consider the better score (out of your original and improvement attempt) in the subject you reappear for when evaluating your eligibility for admission.

https://engineering.careers360.com/articles/jee-main-syllabus

I hope it helps!