Your nervous system is definitely a partially physical limit on how many fields you can tolerate, handle and integrate in a day. A weakened or overwhelmed nervous system will cause you to overextend more quickly (and more painfully). Your brain is, of course, a significant part of that nervous system, but not the whole of it.
There are a number of fields that more holistically address the nervous system as a whole and they do have an effect on improving your energetic throughput and thus how many fields you can use before you slide over your limit.
You’ll also probably find that many of the brain wiring fields will enhance your sensitivity to energy – and becoming more sensitive to energy often goes hand in hand with greater results from fields.
Overall, if you’re trying to increase the number of fields you can handle, ‘growing your brain’ probably isn’t the most efficient way to get there, but it would probably help some.
Check out the Book of Card’s Energy Expansion card or Your Energetic Being for a more direct solution to ‘increasing the number of fields I can handle’.
Surplus energy in the form of Prana, Mana, Force of Life, Chi, Jing, Shen, Ojas and Tejas is also extremely helpful!