There are few constraints to writing music.
Money is not a constraint. You can splash out as much or as little money as you want. You can buy lots of top-notch musical instruments, get a quality microphone, hire an orchestra, get session musicians to play your work, and have the whole thing polished off in a professional recording studio. Alternatively, you can buy a second-hand instrument, play it and sing along at the same time and record the whole piece on a mobile phone. If you have a computer or similar device, the cheapest route of all is to use a free music programme or app.
Time is a constraint, as it is in all walks of life. It takes seconds to generate a musical idea. It might take a few hours to polish and develop it into a finished piece of music or it might take weeks, months, years even.
Decision-making is the biggest constraint to writing music. There are billions of ways to develop a musical idea into a complete piece of music. Literally billions. There are options at every stage in the writing process from the start right through to the end. At any time in the process you will be faced with thousands of possible routes to follow.
Here is an example. You wish to write a short melody with four notes. You can use any of the seven notes in the scale of C major. Any note can be repeated. Every note lasts either one beat or half a beat. Question: how many melodies are there? Answer: 38,416. Reaction: panic attack!
The answer is due to the mathematics of combinations and permutations. There are 7 notes in a major scale and 2,401 ways to combine four of them together (7x7x7x7 = 2,401). There are 16 possible rhythms (2x2x2x2 = 16). In total, there are 38,416 possible melodies (2,401x16 = 38,416).
You can wade through all 38,416 options but it will be a heroic waste of time. The obvious thing to do is to reduce the number of options. Making every note the same duration and disallowing a repeat note, for example, reduces the options from 38,416 to 840 (7x6x5x4 = 840). 840 is a significant reduction over 38,416, but it is still substantially more than the 6 or 7 options you were expecting. There is no way round this, the maths is brutal. It is an absolute racing certainty that you will have lots of options to explore at all stages in writing music.
The only sane solution to making decisions is to use your judgement. When you come across an option you like, keep it, and move swiftly onwards and upwards. This calls for single-minded determination, a willingness to discard things that do not work, and a hefty dose of self-belief. If this is you, then you are a music writer. If you are the sort of person that takes ages to decide whether to have a cup of tea or a cup of coffee in the morning then do not write music. Please, you will only make yourself more miserable.
There is a happy side to all this complexity, the world is never going to run out of music. Never ever. There are so many options available that new music will continue to appear forever.