If I purchase 30 protected IPs and 10 protected times for 3 months, can they be downgraded to 5 protected IPs and 10 protected times?

No. The number of protected IPs and protected times can only be increased but not decreased.

If a protected IP is attacked several times in a day, how will be the consumption of protected times calculated?

Max protection of Anti-DDoS Pro: when the attack traffic to a protected IP in the instance is detected to be at least 10 Gbps and there is no attack traffic after half an hour, the attack will be considered to be over, and one max protection will be deducted; if the attack continues for more than half an hour, one max protection will not be deducted until the attack is over.

What are the differences between the max protection billing of Anti-DDoS Pro and elastic protection billing of Anti-DDoS Advanced?

Anti-DDoS Pro provides max protection of no less than 30 Gbps. When an attack occurs, the maximum DDoS protection capability of Tencent Cloud in the region of the Anti-DDoS Pro instance will be automatically called to provide max protection, which is included in the instance and will not incur additional elastic protection fees.
The elastic protection of Anti-DDoS Advanced is billed by the bandwidth of the elastic protection range corresponding to the maximum attack traffic generated on the day.