I own a domain for my company that is used only to allow receiving CNAME redirects from our customers, eg allow-cname.example.com
You can set that subdomain to be unproxied (grey-clouded ) to not receive CF protections and CNAMEs would then work, but otherwise, no.
Is there any way that I can upload my own SSL certificate to this CNAME and accept the CNAME redirect made by my client using the orange cloud (protected by CF)?
That’s pretty much what the SSL for SAAS product is. You can use a Cloudflare certificate or upload your own and let other domains CNAME to yours with you controlling the settings.
It’s not possible to do this without the mentioned product.
In SSL for SAAS do I pay for each CNAME that will point to my subdomain or do I pay for the subdomain that will receive requests?
It is an Enterprise feature, so you would be looking at custom pricing, which you would need to talk to sales about. As for how it works, you would pay for it on your domain which can then accept CNAMEs from the other domains that you want to manage.