Hi,
We use "“a lot of contact with native speakers helps to learn a second language better "
not " a lot of contacts” because “contact” is an uncountable noun.
The word ‘contact’ in your sentence means the act of communicating with someone and it is in the sense of general. Therefore, it is an uncountable noun. But if you have met and communicated with someone or people who help(s) you frequently, it will become a countable noun. For example: You have a lot of/ several good contacts in India.