I know that with the M103 there are 12 but I would consider replacing them all. I recently replaced all 8 in my M102 (190E) to solve the problem of intermittently noisy lifters during warm-up. They are in-expensive and easy to replace. If one or two are failing now, it is possible that others will follow. I believe replacing all is money well spent.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.