How do you say 20?
Is it pronounced twenty or Tweny?
No. It’s pronounced /ˈtwɛnti/ “twenty” in standard UK English. In US English, as I understand it the later ‘t’ is normally dropped completely – /ˈtwɛni/ “twenny”. This is sometimes heard of Brits using relaxed pronunciation, but is not standard pronunciation and as such would be avoided in serious contexts.
How do the British pronounce twenty?
Break ‘twenty‘ down into sounds: [TWEN] + [TEE] – say it out loud and exaggerate the sounds until you can consistently produce them.