Can you get a check for 20000 and cash it?

If someone is willing to give you a check for 20,000 - sure you can.

20,000 is a reasonably small amount in banking terms. If the check is valid and you have a valid ID to prove you are who you claim to be, the bank should have no problem giving you the cash.

for larger amounts like 2 mil or so, banks will need advance notice to arrange the cash