Seriously, around 5-10 rackets strung at different tensions. You may prefer a higher or lower tension based on weather conditions, balls, or based on how your opponent or you are playing. A few lbs of tension here or there are very noticeable at the higher levels.
You usually have at least one full change of clothes, maybe shoes, a jacket, food and drink.
If the tournament’s stringer doesn’t carry the brand you use you may carry a reel or a more of your own reels with you.
Maybe you have spare towels in there.
Used to carry some basic first aid stuff (plasters, tape, heat spray, painkillers). The pros probably have that too just in case they don’t want to use up an injury timeout to get an Ibuprofen.
It makes sense when you realise they could be on court for maybe 4-5 hours easy.