Once we reach 10 members, we should buy that bottle of whisky to commemorate the occasion.
Once we go over 10 members, why don't we buy that bottle of champagne so we'll always remember the day.
この場合、次のように言えると思います。
ーOnce we reach 10 members, we should buy that bottle of whisky to commemorate the occasion.
「メンバーが１０人に達したら、記念にこのウィスキーを買おうよ」
to reach 10 members で「メンバーが１０人に達する」
ーOnce we go over 10 members, why don't we buy that bottle of champagne so we'll always remember the day.
「メンバーが１０人超えたら、その日の記念にシャンパンを買わない？」
to go over 10 members で「メンバーが１０人を超す」
ご参考まで！
When we get more than 10 members, lets buy that alcohol in commemoration.
"チームメンバーが１０人を超えたらそのお酒を記念に買おうか"
- When we get more than 10 members, lets buy that alcohol in commemoration.
この場合、英語では「お酒」"alcohol" は使いません。どういう酒を具体的に言います。
When we get more than 10 members lets buy that sake to celebrate.
カナダ 
アメリカ合衆国 
