Installing new Loadcenter with a 24" 2 1/2" PVC pipe going to a 2 1/2" Conduit body. How many circuit wires are allowed inside the pipe? Have 3 15 amp circuits and 20 20 amp circuits outgoing.
Each cable is required to be secured to the load center enclosure, NEC 312.5(C). There is an exception that would allow a number of cables to enter through a single section of conduit as you describe, but it has many conditions, so you have to check if you can use it. If you try to use a large 2-1/2" NM clamp in the enclosure to satisfy 312.5(C), the manufacturer of the NM clamp will list a maximum number of cables of a given size that the clamp can secure; I don't know of any that would allow 20 12/2 cables.
If the conduit is entering the top of the enclosure, instead of a conduit body, you could stick a piece of horizontal wireway on top of the conduit. The wireway can have multilple KOs on top allowing each cable to be secured to the wireway with a typical 1/2" or 3/4" NM clamp, which may be listed for 2 or 3 cables per clamp. That would take care of the 312.5(C) issue, and the NM cable can proceed with its sheath intact through the 2-1/2" PVC conduit into the load center enclosure. You'd have to calculate the allowable conduit fill for the 2-1/2" PVC, but if it's under 24" long, you can use up to 60% of the internal cross sectional area. Each NM cable gets treated as if it had a circular cross section of diameter equal to its largest cross sectional dimension.
There's also a serious derating issue if the PVC conduit is over 24" long. You can put at most 9 current carrying conductors in #12 or #14 NM cable inside a conduit longer than 24" while maintaining the usual ratings of a 20A circuit breaker for #12, 15A circuit breaker for #14. So that would mean at most 4 12/2 or 14/2 cables in one conduit. With the wireway approach, you could use multiple smaller conduits between the wireway and the panelboard to keep each conduit down to 4 cables.
So the upshot is see if you meet the rules in 312.5(C) exception; otherwise, each cable needs to be clamped to the enclosure within a clamp that is listed for the number of cables in the clamp. And keep the conduit run to under 24" to avoid derating issues.
Cheers, Wayne