心里没数怎么说用英语

If you don't have a clear count in your mind, you can say "I can't quite put a number on it" or "I don't have an exact count in mind." You can also say "I have a general sense of it, but I'm not sure of the exact amount" or "I don't have a specific number in mind, but I know it's a lot/a little/etc."
For example:
"I can't quite put a number on it, but I think there are a lot of people at the concert."
"I don't have an exact count, but I know there are at least a few dozen people here."
"I have a general sense of it, but I'm not sure of the exact amount. I think there are a lot of books in this library."
"I don't have a specific number in mind, but I know it's a little. There aren't very many people here."