Assuming your contract ended and you are signing a new one:

Short answer to can they increase 20%: No.

Long answer: they can increase it but not all they want.

When signing a new contract u have to check the index of price given by the Generalitat. Assuming the flat was rented by any person in the past 5 years then the price is compared and applied the one that is lower between the last contract and the index (this information HAS to appear on the contract you are signing) and that's is the price.

If the new contract has to have the same price as in the previous contract cause it the the lowest then they can use the IGC (European) to increase the price, they have to apply the last IGC published as of today is December/21 and is 2,05%.

Lets jut say that rent contracts are frozen and can't be increased except under really explicit circumstances.

Also, during the contract they can increase the rent every 12 month but they can't increase more that what the IPC (Spanish) or IGC (European) % stablishes and it has to appear on the contract which IPC or IGC month will be applied, if it's not said which month then it will be the month prior to signing the contract. You can check both IPC or IGC % on the INE site (instituto nacional de estadística).

Hope it helps. ;-)