Taking antibiotics on your own to try to prevent STIs is not advisable. It will not protect against viral infections like HIV, herpes or HPV, may not match the bacteria or doses needed, and contributes to antibiotic resistance - a serious and growing problem, with drug-resistant gonorrhoea being a particular concern.
There are some clinician-supervised preventive strategies for specific bacterial STIs being used in certain higher-risk groups, but these are prescribed and monitored by sexual-health specialists, not something to arrange yourself. They are not a substitute for condoms, vaccination and testing.
If you are worried about repeated STI exposure, the safer approach is to speak to a licensed doctor about appropriate prevention - condoms, HIV PrEP where suitable, HPV and hepatitis B vaccination, and a sensible testing schedule. Never source antibiotics without a prescription or take leftover medicine to prevent or self-treat an infection.