I went to sexual health clinic not long ago, young people should only worry about being infected if they have been with more than 1 person, and thats what the doctor said

While unprotected sex isnt reccomended, as long as you dont go with 2 people and do it theres no chance whatsoever of contracting an STI.
Hmmmm... are you sure that's what the doctor actually said?
It sounds like you may have misinterpreted their advice - generally speaking, if you have only ever had one sexual partner
and that partner has only ever had one partner as well (i.e. you), then unless either of you have contracted an infection from some other means (i.e. a blood transfusion), then you are pretty safe - although unless you have access to your own and your partner's medical records, then you should not assume that you are
completely safe....
But if you only have (and have only ever had) just one sexual partner, then you are categorically
not safe from STD's unless a) you use approriate protection or b) you know exactly the whole sexual history of your partner... and in the real world, most people don't know that, hence why it is always a good idea to use barrier protection (i.e. a condom) with someone until you do know.... (even so, many people may have diseases and not know it). If your partner has had a sexual relationship with
anyone else before you, then you would also need to know the whole sexual relationship of
all of their previous partners too, and so on....
If that is what that doctor really actually said, then I'm very surprised because it's spectacularly bad advice and factually incorrect. But I presume that what they meant was something like above...
Also, preaching abstinence does not work... telling young people that the
best way to avoid catching an STD is by not having sex is a bit like telling a lion that the best way to avoid choking on antelope bones is to not eat antelopes - arguably true, but it isn't going to help one iota. Sorry to sound blunt or crude, but human beings screw and we enjoy it - muchly. The policy of abstinence (like the 'Silver Ring' movement in the US) is fatally flawed, and it's unsafe - not least because of their absurb policy of not advocating the use of condoms. To bonk is human, period. To bonk safely requires educating people, not preaching to them and telling them that they will go to hell and/or catch AIDS if they do...