Depends on what you want to stamp the numbers or letters on to. GH Thomas's Universal Pillar Tool uses a holder for the punch then has the round lathe cross slide dial he is numbering held on an angle bracket below it.Bit more on it here LINKK
But if you are stamping on a flat surface there are other way too. One is to clamp a length of 1.5" angle iron on to the job and hold the punches against the angle iron one at a time as you whack them. You still have to use your own judgement for the spacing between letters or numbers. If you just line all the punches up at once, the spacing looks too wide.
Another way is to run a piece of masking tape along the job then you can feel when the raised letter or number on the punch hits the edge of the tape and so ensures a straight line. But unlike the angle iron, you have to hold the punch square so the letter or number is not lying over at an angle etc.
I've made up various special jigs for special jobs. For stamping numbers on the face of the indexing gear on a GHT Versatile Dividing Head I made a stub to hold the gear flat on a piece of plate, then a piece of angle iron went over the gear to hold the punch in place, with a vertical piece of steel screwed to the angle to hold the punch dead upright. Took longer to make the jig than to stamp the numbers!