i'd go for a minimum of 5 1cm or 1/2" wide holes in the bottom.
when you remove the camellia from its pot try and loosen the compacted roots so that they are able to grow into the new compost.
scrape off any old compost [wash the roots if you find that easier] and repot in good quality ericaceous compost. I also would add some slow release fertilizer granules about a table spoonful.
remove any dead or damaged branches.
keep well watered as it is this time of year that the flower buds for next year are formed.

Also it would benefit from a substantially bigger container.

I was thinking of the black pot in the picture ,it has a bigger base than the one that it’s in, and it is bigger

That should help a lot but eventually can you plan to plant it out in the ground? They will make big shrubs eventually given the room. You can get fertilizers intended fr acid loving plants so feeding with one of those might act as a tonic for now. (this cured yellowing of a rhododendron within a very short time)

That black pot also looks like it is plastic, which means that the roots will suffer more damage from freezing and thawing in the winter. Bubble wrap will be important.

Seaburngirl,Thankyou for your reply ,it is very helpful,
Tugbrethil,Thankyou, we felt a plastic tub may be best this time as it is bigger and so would be lighter to move should we need to, however I do always use bubble wrap in the winter and also fleece if there is a frost.

I think you might have difficulty getting your plant out of the pot because it curves in at the top. If it will not come out easily I would run a sharp knife round the edge and ease it out a bit at a time. You might need a friend to help. If you have fleece or bubble wrap handy I would tie it round the foliage to try and save it from the inevitable man-handling it is going to need. The alternative is to break the pot. Ideally it will be happier in the ground and you can prune it to the size you want after it flowers. I agree with the advice given above.

Thought an update would be appreciated ,we managed to get the Camelia out of the pot, took a lot of persuading and scrapping away the edges of the soil. Couldn’t believe how compacted the soil had become,we needed to soak the whole root ball over 3 days ,scraping away as much soil as we could each day and finally repotting it .It turned out to be a big job,however it is now in its new pot and is now showing bright shiny leaves, so fingers crossed I think it will be fine, Thank you for all advice. It was much appreciated .

Well done, I hope it flowers well for you next year.

thanks for the update. so pleased you managed it successfully.