Is there any way to obtain every 2nd, 3th, 7th etc. packet?

edit

Odd (frame.number & 0x01) and even !(frame.number & 0x01) are pretty easy.

Not pretty but gets the job done.
Adds a new field frame_modulo.remainder. Display filter: frame_modulo.remainder == "0"

-- frame_modulo.lua
-- add field for modulo remainder
-- https://ask.wireshark.org/question/25833/is-there-any-way-to-obtain-every-2nd-3th-7th-etc-packet/

local frame_modulo_info =
{
    version = "1.0.0",
    author = "Chuck Craft",
    description = "Add modulo remainder field. Add menu to request divisor.",
}

set_plugin_info(frame_modulo_info)

-- we create a "protocol" for our tree
local frame_modulo_p = Proto("frame_modulo","Frame number modulo remainder")

-- we create our fields
local frame_modulo_field = ProtoField.string("frame_modulo.remainder", "Frame number modulo remainder")

-- we add our fields to the protocol
frame_modulo_p.fields = { frame_modulo_field }

local modulo_divisor = 1

-- let's do it!
function frame_modulo_p.dissector(tvb,pinfo,root)
    local tree = nil

    -- add our proto if we haven't already
    if not tree then
        tree = root:add(frame_modulo_p)
    end

    tree:add(frame_modulo_field, pinfo.number % modulo_divisor)
end

-- then we register frame_modulo_p as a postdissector
register_postdissector(frame_modulo_p)

-- +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

-- Add new item to Tools menu
local function dialog_menu()
    local function dialog_func(local_divisor)
        local window = TextWindow.new("Modulo divisor");
        local message = string.format("Divisor set to %d. Reload capture file to apply.", local_divisor);
        window:set(message);

        modulo_divisor = local_divisor

    end

    new_dialog("Enter modulo divisor",dialog_func,"Divisor")
end

-- Create the menu entry
register_menu("Modulo packets",dialog_menu,MENU_TOOLS_UNSORTED)

Added in dfilter: Handle arithmetic expressions on the LHS

Available now to test in the Automated Builds