How can I give NEET as my main subject is PCM and additional biology

The qualifying examination for this NEET exam is 12th Standard or its equivalent. The subjects to be included are Physics, Chemistry, Biology/Biotechnology and English. The Candidate should pass all the subjects and the total percentage should be minimum of 50% excluding English from the above-mentioned subjects.
In addition to all the above subjects if you study Mathematics also there will not be any problem. You should have biology in mainstream subjects. Then only you will be eligible for the NEET.
To know more details you can visit the following URL.
https://www.embibe.com/exams/neet-eligibility/

Earlier CBSE was managing the NEET entrance exam but with effect from 2019 it is being conducted by the NTA (National Testing Agency). There are some changes now in the eligibility for NEET exam. NTA has announced it in their revised rules and regulations of NEET.

Earlier it was compulsory to have Biology as the main subject in class 11 and 12 but now NTA has relaxed this condition and students having Biology as additional or supplementary subject can also apply for NEET. General student should have at least 50% aggregate marks in Physics, Chemistry, Biology and English. Another important change is that the upper age limit of 25 years is also removed. The lower age limit is minimum 17 years at the time of admission. One more change in NEET is that now there is no limit to the number of attempts one can make.

Coming to the second part of your query, you have to have Mathematics as the main subject in your class 11 & 12 for applying in JEE.

In order to be eligible to appear in the NEET examination, you need to secure at least 50 percent marks in Physics, Chemistry and Biology apart from English and even in English the qualifying mark would be 50 percent.
The minimum age to appear for the NEET examination has been fixed as 17 years at the time of admission but there is no mention of upper age limit. Earlier there was a capping of 25 years. No of attempts to appear in the NEET examination has been relaxed and now there is no limit in such attempts.
Now in relation to your query regarding JEE examination, you're u must have Physics, Chemistry and Mathematics at the stage of 11 and 12. However, even if qualify JEE test both mains advanced for your inclusion in a prestigious IIT institute, the aggregate - marks of the final Board should be at least 75 percent in your Physics, Chemistry and Mathematics group.

As per National testing agency (NTA), The minimum qualification for appearing in NEET (UG) – 2019 is prescribed in Chapter 3 :

CODE : 01 - A candidate can appear to the qualifying exam who is awaiting for this 12th result. He/she shall not be eligible for the admission to the MBBS or BDS if he/she does not pass the qualifying examination with the required pass percentage of marks at the time of first round of Counselling.

CODE : 02 - The basic requirement is that a candidate must have passed or cleared 10+2 Higher/Senior secondary Examination with Physics, Chemistry, Biology/Biotechnology(PCB) and English as compulsory subjects which shall include practical tests in these subjects. The candidate must have scored an aggregate of 50% (for open category), 45% (for Physically Handicapped -PH), and 40% (for reserved category) in PCB.

Candidates who have passed plus two as private candidate or from Open School are not eligible to appear for NEET. Furthermore, study of Biology/Biotechnology as an Additional Subject at 10+2 level shall not be permissible but the provisions of the regulations disqualifying recognised Open School Board candidates and the candidates who have studied Biology/Biotechnology as and additional Subject has been struck down as per matter of challenge before the Hon'ble High Court of Delhi, Hon'ble High Court of Allahabad, Lucknow Bench and Hon'ble High Court of Madhya Pradesh at Jabalpur.

For more details, you can go through the following link and the FAQs of neet in details. : https://ntaneet.nic.in/Ntaneet/Welcome.aspx