Can't offer any information/advice on SSDI, however the general info is all there online
https://www.ssa.gov/benefits/retirement/planner/credits.html
Is there a way to keep track of all the credits you have?
Register and log into your social security account at https://www.ssa.gov/, it tells you how many work credits you have. Or barring that you could go to your nearest SSA office or try calling Social Security to request that info. Though TBH with all the recent cutbacks/layoffs I'm not too sure how easily you can get help from anyone at the SSA.
EDIT: When you log into your account at ssa.gov it also tells you if you are eligible to apply for disability benefits (and can apply through there).