Ideally you (or a counselor) would have access to GPA/test data for a decent number of prior applicants from your secondary school to that college. That could help you assess whether or not you are in their normal competitive range. The counselor might be able to actually give you an even better assessment that takes into account more factors, including relative rigor.
Absent that, you can just look for their test score and GPA ranges for enrolled students in their CDS submissions. Assuming they provide it, usually I would say if you are above their 50th percentiles you are definitely competitive, and if test optional I would say you could probably go down to 25ths for tests. GPAs are more complicated so you have to realistically self-assess rigor.
This is an eternal debate, but the experienced college counselors at our feederish HS generally suggest 2-3 particularly well-chosen reaches are enough, and past something like 5-6 you are starting to run a real risk of not submitting your best possible applications to any.
Mathematically, part of what is going on here is these are not independent probabilities. So, if similar Reach A does not admit you, your conditional probability of being admitted to similar Reach B should go down. If similar Reach B also does not admit you, then your conditional probability of being admitted to similar Reach C should go down even further. Depending on your model, your conditional probability can get astronomically low very fast.
And another related part is doing the work to understand what are in fact your best shots as reaches. If you really investigate what they are looking for beyond just numbers, and you determine you are in fact what they are looking for, great. If not so much, not so great. So, once you have carefully chosen the ones where you seem most like what they are looking for, the conditional probability all of those will reject you but another school where you are a worse fit will accept you is getting even lower.
As a final thought, the cases where it might be more like 5-6 than 2-3 typically involve either some fundamental indecision about type of school, or maybe a special circumstance with unknown resolution.
So, for example, some kids are applying to both LACs and mid-size private universities, or some are applying to both mid-size private universities and large public universities. 2-3 reaches of each type could get you into that 5-6 range.
Or, maybe you had a significantly worse grade period related to some personal crisis, and you are looking for your reaches to overlook that period for that reason. You might have 2-3 reaches defined on the assumption they will overlook that, and 2-3 different reaches defined on the assumption that they will not.
OK, so our HS also suggests 2 likelies, and 2-3 targets, for most kids. That is like 6-8 total, but again maybe it ends up more like 5-6 for reaches and even targets, which gets you to like 12-14 total. But that is really getting to the outer boundaries now of what they consider advisable, and even then you should have a good reason for why so many.
Still, in the end, none of this is dogmatic. But I think there are good reasons to believe that actually being really thoughtful about your best shots, and then really focusing on writing your best applications, is a better idea than just filling out as many applications as the Common App will allow.
And among them are that if you really have done a good job picking out 2-3, or at most 5-6 reaches, which are your best shots, you can trust it is unlikely adding more will really make a difference in the end.
Indeed, ideally you start realizing some targets or even likelies would be better outcomes anyway than like your 19th-favorite reach. Because that sort of thinking tends to be based on the implicit assumption that more selective = better for you, and that is generally not a good assumption.