Is trinity realistic?

I got: L2D*999998876 + an A in add maths
( the 6 is in English language) at GCSEs and I’m taking maths, FM, physics and music at A-level. Assuming I get predicted 4 A*s by the time I apply, what are the chances I get an interview for maths?

And to add to the above: remember no other college can consider you if Trinity don't interview you. There will be plenty of candidates in Maths and similar subjects they reject before interview that likely would have got one if they'd applied to most other colleges... So that's part of the risk you need to consider.

That is very true.. better to get into my second choice of college than no college at all

Revisit this question after you do proper TMUA preparation and you know roughly how easily you are going to get a very strong score there.
Do those GCSEs "matter"? No. Does a 4A* Prediction matter? No (at least, no in the sense that its not going to help you - its better to get a better prediction of course).
Trinity are going to be on the lookout (pre-interview) for:
1) Candidates who have a proven track record in top-level extended Maths problem solving elsewhere (translation: BMO1 awards, BMO2, perhaps the BPhO, achieved STEP grades (often reapplicants but can be year13s too))
2) Candidates who have the best TMUA marks. Note I'm saying "best" because being simply above average (remember that TMUA isnt just a cambridge admissions exam) could easily be not good enough.
I'm guessing you aren't in #1 which means there will be a huge amount of pressure on #2.
I will be honest and say what I've said a lot of times before; if you are unsure about whether or not you are a strong applicant for Trinity, the answer is that you're not. If you're not, there's going to be some luck involved and there's going to be a good chance you are overlooked or not given full time and consideration. You can sidestep by applying almost anywhere else.

For maths it would be step not tmua

Late reply but it is important to clear up misinformation given these threads can pop up in search engines
No, it is not.
TMUA is the pre-interview test and its the much more important check for why applying to Trinity specifically may be a bad idea. Lets say Trinity is only willing to interview 150 applicants but gets 300+ students applying, and for arguments sake almost all of the students applying are "good enough" on paper for an interview given the only point of comparison is GCSE. It becomes almost a direct competition to be in the top 150 on TMUA out of the Trinity applicants.
The problem is that this simplified scenario ignores the fact that actually like ~50 spots for interview are automatically going to top Olympiad (UK and international) students and someone could claim that its easier to get an offer (not an interview, an offer) from another college than it is to get in the top 100ish out of all the Trinity Maths applicants, given that Trinity does attract a lot more applicants and a higher standard at the top.
If an applicant fails this part of the process, there is no pre-interview pool. You are rejected. You could have had a top 50% TMUA score amongst all applicants, a high enough score to be a serious contender at other colleges. But if you arent in the top X% among the Trinity applicants specifically, thats the end of the process for you.
Once a candidate has a STEP offer, due to the summer pool, it is basically a direct competition to be in the top 250ish out of ~500 offer holders on STEP performance across all colleges. That decides who gets in. In many ways, the most fair part of the process is STEP and the summer pool. Yes, some colleges give out S,1 or S,S offers, but the subtext there is "if you get 1,1 then you will be given a place at another college".