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How many Do 217 night fighters were there?
Short answer
364: of the 1,905 Do 217s built in total, 1,541 were offensive aircraft and the other 364 were night fighters. Production ceased in May 1944. Of the J-1 version, 130 units had been completed by 31 December 1942. The J-2 was obtained only by converting J-1s.

From bomber to night fighter
In 1941, with Germany facing growing night attacks from RAF Bomber Command, it was decided to reinforce the night-fighter force with a version of the Do 217 E. That aircraft, the Do 217 J, received a new "solid" nose, similar to the one Dornier had used on the night-fighter versions of the Do 17 and Do 215, with four 2 cm MG FF cannon and four 7.92 mm forward-firing machine guns. The J-2 version carried the FuG 202 Lichtenstein radar in the nose and had its rear bomb bay plated over. The J-2 was only obtained by converting J-1 airframes.
The production pace
Production began in March 1942, when eight J-1s were built; 13 followed in April, and 55 were built in May. By 31 December 1942 only 130 J-1s had been completed. The BMW 801 that powered the Do 217 J proved underpowered, so a night fighter was produced with the more powerful DB 603 A-1 engines, whose first prototype flew on 31 July 1942.
The total built
Of the 1,905 Do 217s built, 1,541 were offensive aircraft and the other 364 were night fighters. These figures were completed before production ended in May 1944.
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