Increasing Sekki's toughness with a card like Indomitable Will allows Sekki to prevent damage and make tokens even it if has no +1/+1 counters to be removed.

Creature tokens are put onto the battlefield even if there are no more +1/+1 counters to remove. For example, attempting to deal 9 damage to Sekki causes its replacement effect to 1) prevent the 9 damage, 2) remove all eight of the +1/+1 counters, and 3) put nine Spirit creature tokens onto the battlefield.

Damage that can't be prevented still causes counters to be removed from Sekki and puts creature tokens onto the battlefield.

You can sacrifice any eight Spirits to pay for Sekki's activated ability, not just the Spirit creature tokens.