FBI agent Peter Strzok said Thursday he doesn’t know how many witnesses, if any, he interviewed as part of special counsel Robert Mueller’s Russia probe before texting his mistress that President Trump should be impeached. Rep. Trey Gowdy, South Carolina Republican, had asked Mr. Strzok the question early in the embattled FBI agent’s testimony before Congress. However, Mr. Strzok declined to answer citing the advice of FBI counsel. After a brief recess, Mr. Strzok said he was advised that he could answer the question. But when they returned from recess, Mr. Gowdy again asked Mr. Strzok how many witnesses he...