SPF works by checking every domain in the rule. This is called a DNS lookup. So if your SPF record has too many domains in it, it will fail if you have more than 10.
You might see the error Too many lookups or Maximum hop count exceeded.
If you need to add more than 10 lookups to an SPF rule, you can add a subdomain and create a new SPF rule for that subdomain to get around this limit.
Additionally, check with your provider. They might provide a different SPF rule if you're using more than 1 of their services.