quick query
i have a debit card with santander, does anyone know what is biggest transaction i can do on card, ie 5k or 10 k or more ?

It will depend on the account rather than the card, give em a call and ask

also depend on how much money you have.

Card will have a daily limit so call them to check it out.

If there is a limit on your account, you can have it changed if you have the money available in your account. You just need to ring them and let them know in advance.
If its a special purchase such as a car or whatever, they will raise your limit temporarily no problem.