I suppose the answer is yes and no?
I mean, the thing is, when an employer or anyone wants to look at your qualifications, you'll look a LOT better when you do like 8 or 9 subjects and have Grade 7 / A all the way to a Grade 8 or 9 / A* instead of taking a whole range of subjects, but only having C's.
I'm pretty sure that you'll need like 5 GCSE's minimum (C or above, or whatever's the equivalent) for a decent job, but if you're going to aim higher (rock on) then yeah, try to be a specialist would be my advice.
Make sure your qualifications are something you're proud of and try to aim high. Getting high grades, but only taking 8 or so GCSE's is entirely enough for almost all careers in the future. While universities may care about how many subjects you have specialised in they'll probably put a lot more emphasis on the grades you need (they ask you for them in A-level apparently). taking less would also decrease your workload, and so you can focus more on the subjects you want and/or need.
A GCSE Year 10 student. (Good luck for your GCSE's too!)