Trying to ID correct LED bulbs, advice please

 We plan to swap the standard bulbs on our van with LED's, as so many have advised. As usual I get lost in the jargon. The bulbs I've just taken out say, as far as I can tell on the rather wobbly stamp, HP21W12v(E2). Or maybe it's "IHP21W  " etc.

Obviously I want 12v bayonet replacement LED bulbs, but what I don't know is how many of the numbers & letters in the spec have to match, for them to fit, and work!

For example, I get offered "380 BAY15D 1157 WHITE 12 DOME LED DRL SIDELIGHT SIDE LIGHT BULBS SLD200101"  and also "R5W BA15S P21W 1156 9 LED Car Tail"  which also looks the right thing, but again, I don't know. They both fit "standard" car tail lights, but I know from my own car that bayonet fittings and size vary in sometimes annoying ways!

So any advice on how much of these figures on the side of mine need to match would be very welcome please.

As would comments on how many LED's are needed for a useful light one offers "12 Dome LED", one 9 LED, but I'm (can't resist this....) completely in the dark about how many will be either useful or merely a poor glow-worm.

There are lights costing £2.99 a pair, and also ten times that - difficult to know whether it's worth spending the extra, or just a case of being ripped off.

Thanks as ever for all the shared expertise here,

Richard