Currently i am using chef server 12.5.0 version HA opensource and my chef-clients version is 13.7. Post March do we get any support from chef product team or community team for this version mentioned (e.g.,for any bugs or security issues).
For Server 12, there's still no EOL date, so you can keep rolling with that in May. Your client is already EOL tho 
See here: https://docs.chef.io/versions.html#supported-free-distributions
That said, the bins are proprietary but the code's OSS, so there's folks building their own FOSS distros
See https://cinc.sh