In words which end in the sequence <mb> the <b> is always silent. Examples:
bomb climb comb crumb dumb jamb lamb limb numb plumb thumb tomb womb
The addition of a inflectional suffix to words like this makes no difference. The <b> is still silent. Examples:
climber combing dumber
In other words <mb> represents the expected sound sequence mb. Examples:
amber clamber crumble lumber mumble ramble samba symbol timber wombat
Notice that number is ambiguous. It could mean “more numb”. Then the pronunciation would be ˈnʌmə. On the other hand it could mean, well, a number, in which case the pronunciation would of course be ˈnʌmbə.