In swarms; in great numbers — a casual expression describing large crowds or quantities pressing in.
セール初日は客がわんさと押し寄せて店内は大混雑だった。
On the first day of the sale, customers swarmed in and the store was jammed.
景品がわんさか並んでいて、子どもたちは大はしゃぎだった。
Prizes were piled up in great numbers, and the children were very excited.
わんさか／わんさかと人が集まる様子をよく見かける。
You often see people gathering in large numbers.