What is SPF
SPF (Sender Policy Framework) is a DNS TXT record in which the domain owner lists the servers allowed to send mail on its behalf. The receiver compares the sender's IP address with this list and decides whether to trust the message. Without SPF your mail lands in spam more often, and it is easier for scammers to forge your address.
The 10 DNS lookup limit
Every include, a, mx, exists and redirect mechanism makes the receiving server perform an extra DNS lookup, nested ones included. The standard limits them to ten — beyond that the check fails with a PermError and SPF effectively stops working. If the limit is exceeded, remove services you don't use or replace includes with specific ip4/ip6 addresses.
~all or -all
“~all” (softfail) asks receivers to mark mail from other servers as suspicious, “-all” to reject it. Start with ~all, and once you are sure all your services are in the record, switch to -all together with a DMARC policy.