Over the last 7 yrs, I have run the gamut of wired IEM's with cheap buds to wireless with TOTL custom buds.
Current rig is Senn G3 with 1964 A8's.
But I've been thru Carvin and Shure wireless systems and various cheapo buds thru Westone and 1964 Ears customs, as well as extensive use of Aviom and B*ringer P16 systems.
If there's one thing I recommend out of all that - is get the BEST multiple-driver custom-mold buds you can afford, and get a decent wireless system.
As mentioned above, separate the IEM's from the wireless system. Altho they all come with them and are usable, most all-in-one wireless systems include usable (but not great) single-driver buds.
Single driver buds generally don't provide the sound stage nor instrument separation that multiple-driver IEM's do, and that is then compounded by a mono feed. IMHO - at MINIMUM - dual drivers.. preferably triple (for clarity, separation and definition) along with a STEREO IEM mix.
Universal or customs is also your choice.. some get great fit and seal with universals.. Those have never worked for me (small canals) and the customs provide the both fit and seal along with the isolation that lets you monitor at LOWER VOLUMES. IF the seal is bad, you'll need to crank the volume to compensate.
How many drivers is too many? idk! My singer has 12 and loves them, while drummer uses 3, I have 8 and other guitar player has 6.
I look at it this way.. you wouldn't play your Axe gear thru cheap/low-quality monitors or speakers , why would you want to put those in your ear ???