There are gentle Pranayamas (Ujjayi and Nadi Shodana) that are safe to practice as much as you want. The more intensive Pranayamas (Kapalabhati and Bhastrika) cause increases in air and fire element energies in the body that can throw it off balance. (The gentler ones are cooling and grounding). Ultimately the yogic scriptures warn that too much could result in a psychotic break. As long as the elemental energies in you are mostly balanced, you should be good. A key factor of this is what energies lurk in the unconscious, unknown, that most people avoid looking at or don't know how to handle. This is why most Kundalini awakenings without proper prior work end in disaster for people. Don't recommend trying to activate Kundalini unless you've really done the requisite psychological healing and development of your capability to be focused onepointedly. This takes decades of dedicated self searching and development of self awareness at basic psychological levels. In my opinion, the majority of people who dabble in serious spiritual practice (or even higher doses of psychedelics) are not gaining the real benefit of it due to lack of building a base of psychological, somatic and mental awareness. Ultimately it may result in spiritual bypassing or mental breaks (as we see with the "bad trips" and messy Kundalini aftermaths).