What kind of wire do you have installed currently? Are they single wires of different colours or are they sheathed twin and earth type cable? It's normal to put the single wires in some form of conduit. As it's been suggested use the white or yellow PVC pipes. They have a thinner wall than the blue water pipe which makes them easy to bend with a spring tool and all the other accessories and attachments are designed for those pipes. The white pipe is thinner and comes in 2.4m lengths. The yellow is a bit thicker and comes in 4m. I like the white pipe as you can have a slightly shallower wall chase when burying the conduit in the wall but that's not relevant in your case.
Back to current carrying capacity, this seems to be a popular method..
4-6 conductors: Derate by 80%.
7-9 conductors: Derate by 70%.
10-20 conductors: Derate by 50%
This is for unsheathed THW single cables (which is what I'm assuming you have). You don't include the earth/ ground cables in the calculation, only current carrying conductors. Thai Yazaki 2.5mm2 THW cable is rated to 28A in free air. Bangkok Cable 2.5mm2 THW is rated to 29A. So if you had 2-3 circuits of the former (4-6 current carrying conductors) in a single conduit the rating would fall to 22.4A. Which is still fine on a 16 or 20 amp circuit. So don't put too many circuits in one single conduit. You check the current rating of the wire by first finding out the cross sectional area (e.g. 2.5mm2, 4mm2, 6mm2 etc.) and then checking on the manufacturers website but they should all be pretty similar. Check out the tables in the links below.
https://thaiyazaki-electricwire.co.th/images/productindustrial/_2021101206531260227 IEC 01 THW.pdf
https://www.bangkokcable.com/system/product/file_upload/210531_450!750V 70C 60227 IEC 01 (THW)-REV 04.pdf