How many of the 600+ OT laws should we keep?
None. Christ kept them all for me and He did so perfectly.
Romans 10:4 For Christ is the end of the law for righteousness to every one that believeth.
Galatians 3:24-25 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. But after that faith is come, we are no longer under a schoolmaster.
Now, lest I be accused of being an antinomian, I will quote: Galatians 6:2 Bear ye one another's burdens, and so fulfil the law of Christ.
There are plenty of works for the elect to do after regeneration but they are not done for merit or for reward; no, they are done because God ordained the works by His will and for His good pleasure.
Ephesians 2:10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
Philippians 2:13 For it is God which worketh in you both to will and to do of his good pleasure.