▲应当选用复数 have。an unknown number of (数目不详的)与 a certain number of (若干)在用法上相似,修饰可数名词复数,谓语动词用复数。
▲相关例句:
1. It isalso clear that an unknown number of officers have individually, over the years, passed material to loyalist terrorists.
2. Theuprising was put down by the action of the police and the army, and an unknown number of people were killed, among them Muhammadu Marwa Maitatsine, leaderof the fanatical sect which was at the centre of the trouble.
3. However, a certain number of these works are obviously scattered around the world.
4. Once a certain number of cheques have been used, your bank may send you a new book automatically.