Can i write text more than 20 characters?
Yes. Also you can use the script of the transaction to embed some text as Satoshi Nakamoto have done in the genesis block of Bitcoin.
PUSHDATA(65)
[0496b538e853519c726a2c91e61ec11600ae1390813a627c66fb8be7947be63c52da7589379515d4e0a604f8141781e62294721166bf621e73a82cbf2342c858ee]
CHECKSIG
Read more here:
https://en.bitcoin.it/wiki/Script